FRAGMENTS   OF    SCIENCE 
VOL.  I 


FRAGMENTS     OF 
SCIENCE 


A  SERIES  OF  DETACHED 
ESSAYS,  ADDRESSES,  AND  REVIEWS 


BY 

JOHN  TYNDALL,   F.R.S. 

AUTHOR   OF    NEW    FRAGMENTS,   HEAT   A8   A    MODE    OF   MOTION, 

ON   BOUND,   RADIANT    HEAT,   ON    FORMS   OF    WATEK, 

HOURS   OF   EXERCISE   IN  THE   ALPS,   ETC. 


VOL.  I 


NEW   YORK 
D.    APPLETON    AND    COMPANY 

1898 


Authorized  Edition. 


PUBLISHEKS'   NOTE. 


THB  first  edition  of  Prof.  Tyndall's  "  Fragments  of 
Science  "  was  published  some  twenty  years  ago  as  a  sin- 
gle volume,  which  was  made  up  of  a  score  or  more  of 
his  detached  essays,  addresses,  and  reviews.  The  book 
was  afterward  revised,  some  of  the  papers  recast,  and 
from  time  to  time  new  ones  added,  until,  the  size  of  the 
work  becoming  somewhat  unwieldy,  the  present  two- 
volume  edition  was  decided  upon.  This  contains  fif- 
teen additional  papers,  and  represents  the  author's 
latest  changes  and  revisions.  The  volumes  are  uniform 
with  "  New  Fragments,"  recently  issued,  and  the  three 
together  include  all  the  occasional  writings  which 
their  author  has  decided  to  preserve  in  permanent 
form. 


2022122 


PREFACE  TO   THE   SIXTH  EDITION. 


To  avoid  unwieldiness  of  bulk  this  edition  of  the 
'  Fragments '  is  published  in  two  volumes,  instead  of, 
as  heretofore,  in  one. 

The  first  volume  deals  almost  exclusively  with  the 
laws  and  phenomena  of  matter.  The  second  trenches 
upon  questions  in  which  the  phenomena  of  matter  in- 
terlace more  or  less  with  those  of  mind. 

New  Essays  have  been  added,  while  old  ones  have 
been  revised,  and  in  part  recast.  To  be  clear,  without 
being  superficial,  has  been  my  aim  throughout. 

In  neither  volume  have  I  aspired  to  sit  in  the  seat 
of  the  scornful,  but  rather  to  treat  the  questions 
touched  upon  with  a  tolerance,  if  not  a  reverence,  be- 
fitting their  difficulty  and  weight. 

Holding,  as  I  do,  the  nebular  hypothesis,  I  am  logi- 
cally bound  to  deduce  the  life  of  the  world  from  forces 
inherent  in  the  nebula.     With  this  view,  which  is  set 
vii 


viii  PREFACE. 

forth  in  the  second  volume,  it  seemed  but  fair  to  asso- 
ciate the  reasons  which  cause  me  to  conclude  that  every 
attempt  made  in  our  day  to  generate  life  independently 
of  antecedent  life  has  utterly  broken  down. 

A  discourse  on  the  Electric  Light  winds  up  the 
second  volume.  The  incongruity  of  its  position  is  to 
be  referred  to  the  lateness  of  its  delivery. 


CONTENTS  OF  THE  FIRST  VOLUME. 


CHAPTER  PAOK 

I. — THE  CONSTITUTION  OF   NATURE    .....  3 

II.— RADIATION  . 28 

III. — ON  RADIANT  HEAT  IN  RELATION  TO  THE  COLOUR  AND 

CHEMICAL  CONSTITUTION  OF  BODIES      ...  74 

IV. — NEW  CHEMICAL   REACTIONS   PRODUCED   BY  LIGHT          .  96 

V.— THE  SKY 131 

VI.— VOYAGE  TO  ALGERIA  TO  OBSERVE  THE  ECLIPSE       .  142 

VIL— NIAGARA      .                       175 

VIII. — THE   PARALLEL  ROADS   OF  GLEN   ROY           .           .           .  205 

IX. — ALPINE  SCULPTURE     .        .        .        »*     «        .        .  229 

X.— RECENT  EXPERIMENTS  ON  FOG-SIGNALS      .        .        .  253 

XI. — ON  THE  STUDY   OF   PHYSICS 281 

XII.— ON  CRYSTALLINE  AND   SLATY  CLEAVAGE       .           .           .  304 

XIII. — ON   PARAMAGNETIC   AND   DIAMAONETIC   FORCES    .           .  821 

XIV.— PHYSICAL  BASIS  OF  SOLAR  CHEMISTRY       .        .        .  329 

XV. — ELEMENTARY  MAGNETISM 843 

XVI.— ON  FORCE 809 

XVII.— CONTRIBUTIONS  TO  MOLECULAR  PHYSICS     .        .        .  386 
ix 


x  CONTENTS. 

CHAPTER  PAGE 

XVIII.— LIFE   AND  LETTERS   OF   FARADAY  ....      399 

XIX.— THE  COPLEY  MEDALIST  OF  1870  .        .        .        .422 

XX— THE  COPLEY  MEDALIST  OF  1871  .        .        .        .428 

XXI.— DEATH  BY  LIGHTNING        .        .  .        .        .        .439 

XXII.— SCIENCE  AND  THE  SPIRITS  .    444 


MAP 

SHOWING  THE  PARALLEL  ROADS  OF  GLEN  ROY    .    to  face  p.    227 


VOL  I 
INORGANIC   NATURE 


THE  CONSTITUTION  OF  NATURE* 

~TTT~E  cannot  think  of  space  as  finite,  for  wherever 
VV  in  imagination  we  erect  a  boundary,  we  are 
compelled  to  think  of  space  as  existing  beyond  it.  Thus 
by  the  incessant  dissolution  of  limits  we  arrive  at  a 
more  or  less  adequate  idea  of  the  infinity  of  space.  But, 
though  compelled  to  think  of  space  as  unbounded,  there 
is  no  mental  necessity  compelling  us  to  think  of  it 
either  as  filled  or  empty;  whether  it  is  so  or  not  must 
be  decided  by  experiment  and  observation.  That  it  is 
not  entirely  void,  the  starry  heavens  declare;  but  the 
question  still  remains,  Are  the  stars  themselves  hung 
in  vacuo?  Are  the  vast  regions  which  surround  them, 
and  across  which  their  light  is  propagated,  absolutely 
empty?  A  century  ago  the  answer  to  this  question, 
founded  on  the  Newtonian  theory,  would  have  been, 
'  No,  for  particles  of  light  are  incessantly  shot  through 
space/  The  reply  of  modern  science  is  also  negative, 
but  on  different  grounds.  It  has  the  best  possible 
reasons  for  rejecting  the  idea  of  luminiferous  particles; 
but,  in  support  of  the  conclusion  that  the  celestial 
spaces  are  occupied  by  matter,  it  is  able  to  offer  proofs 

* '  Fortnightly  Review,'  1865,  vol.  iii.  p.  129. 
3 


4  FKAGMEXTS    OF    SCIENCE. 

almost  as  cogent  as  those  which  can  be  adduced  of  the 
existence  of  an  atmosphere  round  the  earth.  Men's 
minds,  indeed,  rose  to. a  conception  of  the  celestial  and 
universal  atmosphere  through  the  study  of  the  terres- 
trial and  local  one.  From  the  phenomena  of  sound,  as 
displayed  in  the  air,  they  ascended  to  the  phenomena 
of  light,  as  displayed  in  the  ether;  which  is  the  name 
given  to  the  interstellar  medium. 

The  notion  of  this  medium  must  not  be  considered 
as  a  vague  or  fanciful  conception  on  the  part  of  scientific 
men.  Of  its  reality  most  of  them  are  as  convinced  as 
they  are  of  the  existence  of  the  sun  and  moon.  The 
luminiferous  ether  has  definite  mechanical  properties. 
It  is  almost  infinitely  more  attenuated  than  any  known 
gas,  but  its  properties  are  those  of  a  solid  rather  than 
of  a  gas.  It  resembles  jelly  rather  than  air.  This 
was  not  the  first  conception  of  the  ether,  but  it  is  that 
forced  upon  us  by  a  more  complete  knowledge  of  its 
phenomena.  A  body  thus  constituted  may  have  its 
boundaries;  but,  although  the  ether  may  not  be  co- 
extensive with  space,  it  must  at  all  events  extend  as  far 
as  the  most  distant  visible  stars.  In  fact  it  is  the 
vehicle  of  their  light,  and  without  it  they  could  not 
be  seen.  This  all-pervading  substance  takes  up  their 
molecular  tremors,  and  conveys  them  with  inconceiv- 
able rapidity  to  our  organs  of  vision.  It  is  the  trans- 
ported shiver  of  bodies  countless  millions  of  miles  dis- 
tant, which  translates  itself  in  human  consciousness 
into  the  splendour  of  the  firmament  at  night. 

If  the  ether  have  a  boundary,  masses  of  ponderable 
matter  might  be  conceived  to  exist  beyond  it,  but  they 
could  emit  no  light.  Beyond  the  ether  dark  suns 
might  burn;  there,  under  proper  conditions,  combus- 
tion might  be  carried  on;  fuel  might  consume  unseen, 
and  metals  be  fused  in  invisible  fires.  A  body,  more- 


THE    CONSTITUTION    OF    NATUKE.  5 

over,  once  heated  there,  would  continue  for  ever  heated ; 
a  sun  or  planet  once  molten,  would  continue  for  ever 
molten.  For,  the  loss  of  heat  being  simply  the  ab- 
straction of  molecular  motion  by  the  ether,  where  this 
medium  is  absent  no  cooling  could  occur.  A  sentient 
being  on  approaching  a  heated  body  in  this  region, 
would  be  conscious  of  no  augmentation  of  temperature. 
The  gradations  of  warmth  dependent  on  the  laws  of 
radiation  would  not  exist,  and  actual  contact  would 
first  reveal  the  heat  of  an  extra  ethereal  sun. 

Imagine  a  paddle-wheel  placed  in  water  and  caused 
io  rotate.  From  it,  as  a  centre,  waves  would  issue  in 
all  directions,  and  a  wader  as  he  approached  the  place 
of  disturbance  would  be  met  by  stronger  and  stronger 
waves.  This  gradual  augmentation  of  the  impression 
made  upon  the  wader  is  exactly  analogous  to  the  aug- 
mentation of  light  when  we  approach  a  luminous 
source.  In  the  one  case,  however,  the  coarse  common 
nerves  of  the  body  suffice;  for  the  other  we  must  have 
the  finer  optic  nerve.  But  suppose  the  water  with- 
drawn; the  action  at  a  distance  would  then  cease,  and, 
as  far  as  the  sense  of  touch  is  concerned,  the  wader 
would  be  first  rendered  conscious  of  the  motion  of  the 
wheel  by  the  blow  of  the  paddles.  The  transference  of 
motion  from  the  paddles  to  the  water  is  mechanically 
similar  to  the  transference  of  molecular  motion  from 
the  heated  body  to  the  ether;  and  the  propagation  of 
waves  through  the  liquid  is  mechanically  similar  to  the 
propagation  of  light  and  radiant  heat. 

As  far  as  our  knowledge  of  space  extends,  we  are 
to  conceive  it  as  the  holder  of  the  luminiferous  ether, 
through  which  are  interspersed,  at  enormous  distances 
apart,  the  ponderous  nuclei  of  the  stars.  Associated 
with  the  star  that  most  concerns  us  we  have  a  group 
of  dark  planetary  masses  revolving  at  various  distances 


6  FRAGMENTS    OF    SCIENCE. 

round  it,  each  again  rotating  on  its  own  axis;  and, 
finally,  associated  with  some  of  these  planets  we  have 
dark  bodies  of  minor  note — the  moons.  Whether  the 
other  fixed  stars  have  similar  planetary  companions  or 
not  is  to  \is  a  matter  of  pure  conjecture,  which  may  or 
may  not  enter  into  our  conception  of  the  universe.  But 
probably  every  thoughtful  person  believes,  with  regard 
to  those  distant  suns,  that  there  is,  in  space,  something 
besides  our  system  on  which  they  shine. 

From  this  general  view  of  the  present  condition  of 
space,  and  of  the  bodies  contained  in  it,  we  pass  to  the 
enquiry  whether  things  were  so  created  at  the  begin- 
ning. Was  space  furnished  at  once,  by  the  fiat  of  Om- 
nipotence, with  these  burning  orbs?  In  presence  of 
the  revelations  of  science  this  view  is  fading  more  and 
more.  Behind  the  orbs,  we  now  discern  the  nebulae 
from  which  they  have  been  condensed.  And  without 
going  so  far  back  as  the  nebulae,  the  man  of  science  can 
prove  that  out  of  common  non-luminous  matter  this 
whole  pomp  of  stars  might  have  been  evolved. 

The  law  of  gravitation  enunciated  by  Newton  is, 
that  every  particle  of  matter  in  the  universe  attracts 
every  other  particle  with  a  force  which  diminishes  as 
the  square  of  the  distance  increases.  Thus  the  sun  and 
the  earth  mutually  pull  each  other;  thus  the  earth  and 
the  moon  are  kept  in  company;  the  force  which  holds 
every  respective  pair  of  masses  together  being  the  in-- 
tegrated  force  of  their  component  parts.  Under  the 
'  operation  of  this  force  a  stone  falls  to  the  ground  and 
is  warmed  by  the  shock;  under  its  operation  meteors 
plunge  into  our  atmosphere  and  rise  to  incandescence. 
Showers  of  such  meteors  doubtless  fall  incessantly  upon 
the  sun.  Acted  on  by  this  force,  the  earth,  were  it 
stopped  in  its  orbit  to-morrow,  would  rush  towards,  and 
finally  combine  with,  the  sun.  Heat  would  also  be 


THE    CONSTITUTION    OF    NATURE.  7 

developed  by  this  collision.  Mayer  first,  and  Helm- 
holtz  and  Thomson  afterwards,  have  calculated  its 
amount.  It  would  equal  that  produced  by  the  com- 
bustion of  more  than  5,000  worlds  of  solid  coal,  all 
this  heat  being  generated  at  the  instant  of  collision. 
In  the  attraction  of  gravity,  therefore,  acting  upon 
non-luminous  matter,  we  have  a  source  of  heat  more 
powerful  than  could  be  derived  from  any  terrestrial 
combustion.  And  were  the  matter  of  the  universe 
thrown  in  cold  detached  fragments  into  space,  and  there 
abandoned  to  the  mutual  gravitation  of  its  own  parts, 
the  collision  of  the  fragments  would  in  the  end  pro- 
duce the  fires  of  the  stars. 

The  action  of  gravity  upon  matter  originally  cold 
may,  in  fact,  be  the  origin  of  all  light  and  heat,  and 
also  the  proximate  source  of  such  other  powers  as  are 
generated  by  light  and  heat.  But  we  have  now  to 
enquire  what  is  the  light  and  what  is  the  heat  thus 
produced?  This  question  has  already  been  answered 
in  a  general  way.  Both  light  and  heat  are  modes  of 
motion.  Two  planets  clash  and  come  to  rest;  their 
motion,  considered  as  that  of  masses,  is  destroyed,  but 
it  is  in  great  part  continued  as  a  motion  of  their  ul- 
timate particles.  It  is  this  latter  motion,  taken  up 
by  the  ether,  and  propagated  through  it  with  a  velo- 
city of  186,000  miles  a  second,  that  comes  to  us  as 
the  light  and  heat  of  suns  and  stars.  The  atoms  of  a 
hot  body  swing  with  inconceivable  rapidity — billions  of 
times  in  a  second — but  this  power  of  vibration  neces- 
sarily implies  the  operation  of  forces  between  the  atoms 
themselves.  It  reveals  to  us  that  while  they  are  held 
together  by  one  force,  they  are  kept  asunder  by  another, 
their  position  at  any  moment  depending  on  the  equilib- 
rium of  attraction  and  repulsion.  The  atoms  behave 
as  if  connected  by  elastic  springs,  which  oppose  at  the 


8  FRAGMENTS    OF    SCIENCE. 

same  time  their  approach  and  their  retreat,  but  which 
tolerate  the  vibration  called  heat.  The  molecular  vi- 
bration once  set  up  is  instantly  shared  with  the  ether, 
and  diffused  by  it  throughout  space. 

We  on  the  earth's  surface  live  night  and  day  in  the 
midst  of  ethereal  commotion.  The  medium  is  never 
still.  The  cloud  canopy  above  us  may  be  thick  enough 
to  shut  out  the  light  of  the  stars;  but  this  canopy  is 
itself  a  warm  body,  which  radiates  its  thermal  motion 
through  the  ether.  The  earth  also  is  warm,  and  sends 
its  heat-pulses  incessantly  forth.  It  is  the  waste  of  its 
molecular  motion  in  space  that  chills  the  earth  upon  a 
clear  night;  it  is  the  return  of  thermal  motion  from 
the  clouds  which  prevents  the  earth's  temperature,  on 
a  cloudy  night,  from  falling  so  low.  To  the  concep- 
tion of  space  being  filled,  we  must  therefore  add 
the  conception  of  its  being  in  a  state  of  incessant 
tremor. 

The  sources  of  this  vibration  are  the  ponderable 
masses  of  the  universe.  Let  us  take  a  sample  of  these 
and  examine  it  in  detail.  When  we  look  to  our  planet, 
we  find  it  to  be  an  aggregate  of  solids,  liquids,  and 
gases.  Subjected  to  a  sufficiently  low  temperature,  the 
two  last  would  also  assume  the  solid  form.  When  we 
look  at  any  one  of  these,  we  generally  find  it  composed 
of  still  more  elementary  parts.  We  learn,  for  example, 
that  the  water  of  our  rivers  is  formed  by  the  union,  in 
definite  proportions,  of  two  gases,  oxygen  and  hydrogen. 
We  know  how  to  bring  these  constituents  together,  so 
as  to  form  water:  we  also  know  how  to  analyse  the  water, 
and  recover  from  it  its  two  constituents.  So,  likewise, 
as  regards  the  solid  portions  of  the  earth.  Our  chalk 
hills,  for  example,  are  formed  by  a  combination  of  car- 
bon, oxygen,  and  calcium.  These  are  the  so-called 
.elements  the  union  of  which,  in  definite  proportions,  has 


THE    CONSTITUTION    OF    NATUEE.  9 

resulted  in  the  formation  of  chalk.  The  flints  within 
the  chalk  we  know  to  be  a  compound  of  oxygen  and 
silicium,  called  silica;  and  our  ordinary  clay  is,  for  the 
most  part,  formed  by  the  union  of  silicium,  oxygen,  and 
the  well-known  light  metal,  aluminium.  By  far  the 
greater  portion  of  the  earth's  crust  is  compounded 
of  the  elementary  substances  mentioned  in  these  few 
lines. 

The  principle  of  gravitation  has  been  already  de- 
scribed as  an  attraction  which  every  particle  of  matter, 
however  small,  exerts  on  every  other  particle.  With 
gravity  there  is  no  selection;  no  particular  atoms  choose, 
by  preference,  other  particular  atoms  as  objects  of  at- 
traction; the  attraction  of  gravitation  is  proportional 
simply  to  the  quantity  of  the  attracting  matter,  regard- 
less of  its  quality.  But  in  the  molecular  world  which 
we  have  now  entered  matters  are  otherwise  arranged. 
Here  we  have  atoms  between  which  a  strong  attraction 
is  exercised,  and  also  atoms  between  which  a  weak  attrac- 
tion is  exercised.  One  atom  can  jostle  another  out  of 
its  place,  in  virtue  of  a  superior  force  of  attraction. 
But,  though  the  amount  of  force  exerted  varies  thus 
from  atom  to  atom,  it  is  still  an  attraction  of  the  same 
mechanical  quality,  if  I  may  use  the  term,  as  that  of 
gravity  itself.  Its  intensity  might  be  measured  in  the 
same  way,  namely  by  the  amount  of  motion  which  it 
can  generate  in  a  certain  time.  Thus  the  attraction  of 
gravity  at  the  earth's  surface  is  expressed  by  the  number 
32;  because,  when  acting  freely  on  a  body  for  a  second 
of  time,  gravity  imparts  to  the  body  a  velocity  of  thirty- 
two  feet  a  second.  In  like  manner  the  mutual  attrac- 
tion of  oxygen  and  hydrogen  might  be  measured  by  the 
velocity  imparted  to  the  atoms  in  their  rushing  to- 
gether. Of  course  such  a  unit  of  time  as  a  second  is  not 
here  to  be  thought  of,  the  whole  interval  required  by 


10  FEAGMENTS    OF    SCIENCE. 

the  atoms  to  cross  the  minute  spaces  which  separate 
them  amounting  only  to  an  inconceivably  small  fraction 
of  a  second. 

It  has  been  stated  that  when  a  body  falls  to  the 
earth  it  is  warmed  by  the  shock.  Here,  to  use  the  ter- 
minology of  Mayer,  we  have  a  mechanical  combination 
of  the  earth  and  the  body.  Let  us  suffer  the  falling 
body  and  the  earth  to  dwindle  in  imagination  to  the 
size  of  atoms,  and  for  the  attraction  of  gravity  let  us 
substitute  that  of  chemical  affinity;  we  have  then  what 
is  called  a  chemical  combination.  The  effect  of  the 
union  in  this  case  also  is  the  development  of  heat,  and 
from  the  amount  of  heat  generated  we  can  infer  the 
intensity  of  the  atomic  pulL  Measured  by  ordinary 
mechanical  standards  this  is  enormous.  Mix  eight 
pounds  of  oxygen  with  one  of  hydrogen,  and  pass  a 
spark  through  the  mixture;  the  gases  instantly  com- 
bine, their  atoms  rushing  over  the  little  distances  which 
separate  them.  Take  a  weight  of  47,000  pounds  to  an 
elevation  of  1,000  feet  above  the  earth's  surface,  and  let 
it  fall;  the  energy  with  which  it  will  strike  the  earth 
will  not  exceed  that  of  the  eight  pounds  of  oxygen 
atoms,  as  they  dash  against  one  pound  of  hydrogen 
atoms  to  form  water. 

It  is  sometimes  stated  that  gravity  is  distinguished 
from  all  other  forces  by  the  fact  of  its  resisting  conver- 
sion into  other  forms  of  force.  Chemical  affinity,  it  is 
said,  can  be  converted  into  heat  and  light,  and  these 
again  into  magnetism  and  electricity:  but  gravity  re- 
fuses to  be  so  converted;  being  a  force  maintaining 
itself  under  all  circumstances,  and  not  capable  of  dis- 
appearing to  give  place  to  another.  The  statement 
arises  from  vagueness  of  thought.  If  by  it  be  meant 
that  a  particle  of  matter  can  never  be  deprived  of  its 
weight,  the  assertion  is  correct;  but  the  law  which  af- 


THE    CONSTITUTION    OF    NATURE.  H 

firms  the  convertibility  of  natural  forces  was  never  in- 
tended, in  the  minds  of  those  who  understood  it,  to 
affirm  that  such  a  conversion  as  that  here  implied  occurs 
in  any  case  whatever.  As  regards  convertibility  into 
heat,  gravity  and  chemical  affinity  stand  on  precisely 
the  same  footing.  The  attraction  in  the  one  .case  is  as 
indestructible  as  in  the  other.  Nobody  affirms  that 
when  a  stone  rests  upon  the  surface  of  the  earth,  the 
mutual  attraction  of  the  earth  and  stone  is  abolished; 
nobody  means  to  affirm  that  the  mutual  attraction  of 
oxygen  for  hydrogen  ceases,  after  the  atoms  have  com- 
bined to  form  water.  What  is  meant,  in  the  case  of 
chemical  affinity,  is,  that  the  pull  of  that  affinity,  acting 
through  a  certain  space,  imparts  a  motion  of  translation 
of  the  one  atom  towards  the  other.  This  motion  is  not 
heat,  nor  is  the  force  that  produces  it  heat.  But  when 
the  atoms  strike  and  recoil,  the  motion  of  translation  is 
converted  into  a  motion  of  vibration,  which  is  heat. 
The  vibration,  however,  so  far  from  causing  the  extinc- 
tion of  the  original  attraction,  is  in  part  carried  on  by 
that  attraction.  The  atoms  recoil,  in  virtue  of  the 
elastic  force  which  opposes  actual  contact,  and  in  the 
recoil  they  are  driven  too  far  back.  The  original  at- 
traction then  triumphs  over  the  force  of  recoil,  and 
urges  the  atoms  once  more  together.  Thus,  like  a 
pendulum,  they  oscillate,  until  their  motion  is  imparted 
to  the  surrounding  aether;  or,  in  other  words,  until 
their  heat  becomes  radiant  heat. 

In  this  sense,  and  in  this  sense  only,  is  chemical 
affinity  converted  into  heat.  There  is,  first  of  all,  the 
attraction  between  the  atoms;  there  is,  secondly,  space 
between  them.  Across  this  space  the  attraction  urges 
them.  They  collide,  they  recoil,  they  oscillate.  There 
is  here  a  change  in  the  form  of  the  motion,  but  there  is 
no  real  loss.  It  is  so  with  the  attraction  of  gravity. 


12  FKAGMENTS    OF    SCIENCE. 

To  produce  motion  by  gravity  space  must  also  inter- 
vene between  the  attracting  bodies.  When  they  strike 
together  motion  is  apparently  destroyed,  but  in  reality 
there  is  no  destruction.  Their  atoms  are  suddenly  urged 
together  by  the  shock;  by  their  own  perfect  elasticity 
these  atoms  recoil;  and  thus  is  set  up  the  molecular 
oscillation  which,  when  communicated  to  the  proper 
nerves,  announces  itself  as  heat. 

It  was  formerly  universally  supposed  that  by  the 
collision  of  unelastic  bodies  force  was  destroyed.  Men 
saw,,  for  example,  that  when  two  spheres  of  clay, 
painter's  putty,  or  lead  for  example,  were  urged  to- 
gether, the  motion  possessed  by  the  masses,  prior  to  im- 
pact, was  more  or  less  annihilated.  They  believed  in 
an  absolute  destruction  of  the  force  of  impact.  Until 
recent  times,  indeed,  no  difficulty  was  experienced  in 
believing  this,  whereas,  at  present,  the  ideas  of  force 
and  it&  destruction  refuse  to  be  united  in  most  philo- 
sophic minds.  In  the  collision  of  elastic  bodies,  on  the 
contrary,  it  was  observed  that  the  motion  with  which 
they  clashed  together  was  in  great  part  restored  by  the 
resiliency  of  the  masses,  the  more  perfect  the  elasticity 
the  more  complete  being  the  restitution.  This  led  to 
the  idea  of  perfectly  elastic  bodies — bodies  competent 
to  restore  by  their  recoil  the  whole  of  the  motion  which 
they  possessed  before  impact — and  this  again  to  the  idea 
of  the  conservation  of  force,  as  opposed  to  that  destruc- 
tion of  force  which  was  supposed  to  occur  when  un- 
elastic bodies  met  in  collision. 

We  now  know  that  the  principle  of  conservation 
holds  equally  good  with  elastic  and  unelastic  bodies. 
Perfectly  elastic  bodies  would  develop  no  heat  on  col- 
lision. They  would  retain  their  motion  afterwards, 
though  its  direction  might  be  changed;  and  it  is  only 
when  sensible  motion  is  wholly  or  partly  destroyed, 


THE    CONSTITUTION    OF    NATURE.  13 

that  heat  is  generated.  This  always  occurs  in  unelastic 
collision,  the  heat  developed  being  the  exact  equivalent 
of  the  sensible  motion  extinguished.  This  heat  virtu- 
ally declares  that  the  property  of  elasticity,  denied  to  the 
masses,  exists  among  their  atoms;  by  the  recoil  and  os- 
cillation of  which  the  principle  of  conservation  is 
vindicated. 

But  ambiguity  in  the  use  of  the  term  '  force '  makes 
itself  more  and  more  felt  as  we  proceed.  We  have 
called  the  attraction  of  gravity  a  force,  without  any 
reference  to  motion.  A  body  resting  on  a  shelf  is  as 
much  pulled  by  gravity  as  when,  after  having  been 
pushed  off  the  shelf,  it  falls  towards  the  earth.  We 
applied  the  term  force  also  to  that  molecular  attraction 
which  we  called  chemical  affinity.  When,  however,  we 
spoke  of  the  conservation  of  force,  in  the  case  of  elastic 
collision,  we  meant  neither  a  pull  nor  a  push,  which,  as 
just  indicated,  might  be  exerted  upon  inert  matter,  but 
we  meant  force  invested  in  motion — the  vis  viva,  as  it 
is  called,  of  the  colliding  masses. 

Force  in  this  form  has  a  definite  mechanical  mea- 
sure, in  the  amount  of  work  that  it  can  perform.  The 
simplest  form  of  work  is  the  raising  of  a  weight.  A 
man  walking  up-hill,  or  up-stairs,  with  a  pound  weight 
in  his  hand,  to  an  elevation  say  of  sixteen  feet,  performs 
a  certain  amount  of  work,  over  and  above  the  lifting  of 
his  own  body.  If  he  carries  the  pound  to  a  height  of 
thirty-two  feet,  he  does  twice  the  work;  if  to  a  height 
of  forty-eight  feet,  he  does  three  times  the  work;  if  to 
sixty-four  feet,  he  does  four  times  the  work,  and  so  on. 
If,  moreover,  he  carries  up  two  pounds  instead  of  one, 
other  things  being  equal,  he  does  twice  the  work;  if 
three,  four,  or  five  pounds,  he  does  three,  four,  or  five 
times  the  work.  In  fact,  it  is  plain  that  the  work  per- 
formed depends  on  two  factors,  the  weight  raised  and 


14  FEAGMENTS    OF    SCIENCE. 

the  height  to  which  it  is  raised.  It  is  expressed  by  the 
product  of  these  two  factors. 

But  a  body  may  be  caused  to  reach  a  certain  ele-- 
vation  in  opposition  to  the  force  of  gravity,  without  be- 
ing actually  carried  up.  If  a  hodman,  for  example, 
wished  to  land  a  brick  at  an  elevation  of  sixteen  feet 
above  the  place  where  he  stood,  he  would  probably 
pitch  it  up  to  the  bricklayer.  He  would  thus  impart, 
by  a  sudden  effort,  a  velocity  to  the  brick  sufficient  to 
raise  it  to  the  required  height;  the  work  accomplished 
by  that  effort  being  precisely  the  same  as  if  he  had 
slowly  carried  up  the  brick.  The  initial  velocity  to  be 
imparted,  in  this  case,  is  well  known.  To  reach  a 
height  of  sixteen  feet,  the  brick  must  quit  the  man's 
hand  with  a  velocity  of  thirty-two  feet  a  second.  It  is 
needless  to  say,  that  a  body  starting  with  any  velocity, 
would,  if  wholly  unopposed  or  unaided,  continue  to 
move  for  ever  with  the  same  velocity.  But  when,  as 
in  the  case  before  us,  the  body  is  thrown  upwards,  it 
moves  in  opposition  to  gravity,  which  incessantly  re- 
tards its  motion,  and  finally  brings  it  to  rest  at  an  ele- 
vation of  sixteen  feet.  If  not  here  caught  by  the  brick- 
layer, it  would  return  to  the  hodman  with  an  accelerated 
motion,  and  reach  his  hand  with  the  precise  velocity  it 
possessed  on  quitting  it. 

An  important  relation  between  velocity  and  work  is 
here  to  be  pointed  out.  Supposing  the  hodman  com- 
petent to  impart  to  the  brick,  at  starting,  a  velocity  of 
sixty-four  feet  a  second,  or  twice  its  former  velocity, 
would  the  amount  of  work  performed  be  twice  what  it 
was  in  the  first  instance?  No;  it  would  be  four  times 
that  quantity;  for  a  body  starting  with  twice  the  velo- 
city of  another,  will  rise  to  four  times  the  height.  In 
like  manner,  a  three-fold  velocity  will  give  a  nine-fold 
elevation,  a  four-fold  velocity  will  give  a  sixteen-fold 


THE   CONSTITUTION    OF   NATURE.  15 

elevation,  and  so  on.  The  height  attained,  then,  is  not 
proportional  to  the  initial  velocity,  but  to  the  square  of 
the  velocity.  As  before,  the  work  is  also  proportional 
to  the  weight  elevated.  Hence  the  work  which  any 
moving  mass  whatever  is  competent  to  perform,  in 
virtue  of  the  motion  which  it  at  any  moment  possesses, 
is  jointly  proportional  to  its  weight  and  the  square  of  its 
velocity.  Here,  then,  we  have  a  second  measure  of 
work,  in  which  we  simply  translate  the  idea  of  height 
into  its  equivalent  idea  of  motion. 

In  mechanics,  the  product  of  the  mass  of  a  moving 
body  into  the  square  of  its  velocity,  expresses  what  is 
called  the  vis  viva,  or  living  force.  It  is  also  sometimes 
called  the  '  mechanical  effect/  If,  for  example,  a  can- 
non pointed  to  the  zenith  urge  a  ball  upwards  with  twice 
the  velocity  imparted  to  a  second  ball,  the  former  will 
rise  to  four  times  the  height  attained  by  the  latter.  If 
directed  against  a  target,  it  will  also  do  four  times  the 
execution.  Hence  the  importance  of  imparting  a  high 
velocity  to  projectiles  in  war.  Having  thus  cleared  our 
way  to  a  perfectly  definite  conception  of  the  vis  viva  of 
moving  masses,  we  are  prepared  for  the  announcement 
that  the  heat  generated  by  the  shock  of  a  falling  body 
against  the  earth  is  proportional  to  the  vis  viva  annihil- 
ated. The  heat  is  proportional  to  the  square  of  the 
velocity.  In  the  case,  therefore,  of  two  cannon-balls 
of  equal  weight,  if  one  strike  a  target  with  twice  the 
velocity  of  the  other,  it  will  generate  four  times  the 
heat,  if  with  three  times  the  velocity,  it  will  generate 
nine  times  the  heat,  and  so  on. 

Mr.  Joule  has  shown  that  a  pound  weight  falling 
from  a  height  of  772  feet,  or  772  pounds  falling  through 
one  foot,  will  generate  by  its  collision  with  the  earth 
an  amount  of  heat  sufficient  to  raise  a  pound  of  water 
one  degree  Fahrenheit  in  temperature.  772  "  foot- 


16  FBAGMENTS    OF    SCIENCE. 

pounds  "  constitute  the  mechanical  equivalent  of  heat. 
Now,  a  body  falling  from  a  height  of  772  feet,  has,  upon 
striking  the  earth,  a  velocity  of  223  feet  a  second;  and 
if  this  velocity  were  imparted  to  the  body,  by  any  other 
means,  the  quantity  of  heat  generated  by  the  stoppage 
of  its  motion  would  be^that  stated  above.  '  Six  times 
that  velocity,  or  1,338  feet,  would  not  be  an  inordinate 
one  fgr  a  cannon-ball  as  it  quits  the  gun.  Hence,  a 
cannon-ball  moving  with  a  velocity  of  1,338  feet  a  sec- 
ond, wo  aid,  by  collision,  generate  an  amount  of  heat 
competent  to  raise  its  own  weight  of  water  36  degrees 
Fahrenheit  in  temperature.  If  composed  of  iron,  and 
if  all  the  heat  generated  were  concentrated  in  the  ball 
itself,  its  temperature  would  be  raised  about  360  de- 
grees Fahrenheit;  because  one  degree  in  the  case  of 
water  is  equivalent  to  about  ten  degrees  in  the  case  of 
iron.  In  artillery  practice,  the  heat  generated  is  usu- 
ally concentrated  upon  the  front  of  the  bolt,  and  on  the 
portion  of  the  target  first  struck.  By  this  concentra- 
tion the  heat  developed  becomes  sufficiently  intense  to 
raise  the  dust  of  the  metal  to  incandescence,  a  flash  of 
light  often  accompanying  collision  with  the  target. 

Let  us  now  fix  our  attention  for  a  moment  on  the 
gunpowder  which  urges  the  cannon-ball.  This  is  com- 
posed of  combustible  matter,  which  if  burnt  in  the  open 
air  would  yield  a  certain  amount  of  heat.  It  will  not 
yield  this  amount  if  it  perform  the  work  of  urging  a 
ball.  The  heat  then  generated  by  the  gunpowder  will 
fall  short  of  that  produced  in  the  open  air,  by  an  amount 
equivalent  to  the  vis  viva  of  the  ball;  and  this  exact 
amount  is  restored  by  the  ball  on  its  collision  with  the 
target.  In  this  perfect  way  are  heat  and  mechanical 
motion  connected. 

Broadly  enunciated,  the  principle  of  the  conserva- 
tion of  force  asserts,  that  the  quantity  of  force  in  the 


THE    CONSTITUTION    OF    NATURE.  17 

universe  is  as  unalterable  as  the  quantity  of  matter; 
that  it  is  alike  impossible  to  create  force  and  to  anni- 
hilate it.  But  in  what  sense  are  we  to  understand  this 
assertion?  It  would  be  manifestly  inapplicable  to  the 
force  of  gravity  as  denned  by  Newton;  for  this  is  a 
force  varying  inversely  as  the  square  of  the  distance; 
and  to  affirm  the  constancy  of  a  varying  force  would  be 
self -contradictory.  Yet,  when  the  question  is  properly 
understood,  gravity  forms  no  exception  to  the  law  of 
conservation.  Following  the  method  pursued  by  Helm- 
holtz,  I  will  here  attempt  an  elementary  exposition  of 
this  law.  Though  destined  in  its  applications  to  pro- 
duce momentous  changes  in  human  thought,  it  is  not 
difficult  of  comprehension. 

For  the  sake  of  simplicity  we  will  consider  a  particle 
of  matter,  which  we  may  call  F,  to  be  perfectly  fixed, 
and  a  second  movable  particle,  D,  placed  at  a  distance 
from  F.  We  will  assume  that  these  two  particles  at- 
tract each  other  according  to  the  Newtonian  law.  At  a 
certain  distance,  the  attraction  is  of  a  certain  definite 
amount,  which  might  be  determined  by  means  of  a 
spring  balance.  At  half  this  distance  the  attraction 
would  be  augmented  four  times;  at  a  third  of  the  dis- 
tance, nine  times;  at  one-fourth  of  the  distance,  sixteen 
times,  and  so  on.  In  every  case,  the  attraction  might 
be  measured  by  determining,  with  the  spring  balance, 
the  amount  of  tension  just  sufficient  to  prevent  D  from 
moving  towards  F.  Thus  far  we  have  nothing  what- 
ever to  do  with  motion;  we  deal  with  statics,  not  with 
dynamics.  We  simply  take  into  account  the  distance  of 
D  from  F,  and  the  pull  exerted  by  gravity  at  that  dis- 
tance. 

It  is  customary  in  mechanics  to  represent  the  mag- 
nitude of  a  force  by  a  line  of  a  certain  length,  a  force  of 
double  magnitude  being  represented  by  a  line  of  double 


18  FKAGMENTS    OF    SCIENCE. 

length,  and  so  on.  Placing  then  the  particle  D  at  a  dis- 
tance from  F,  we  can,  in  imagination,  draw  a  straight 
line  from  D  to  F,  and  at  D  erect  a  perpendicular  to  this 
line,  which  shall  represent  the  amount  of  the  attraction 
exerted  on  D.  If  D  be  at  a  very  great  distance  from  F, 
the  attraction  will  be  very  small,  and  the  perpendicular 
consequently  very  short.  If  the  distance  be  practically 
infinite,  the  attraction  is  practically  nil.  Let  us  now 
suppose  at  every  point  in  the  line  joining  F  and  D  a  per- 
pendicular to  be  erected,  proportional  in  length  to  the 
attraction  exerted  at  that  point;  we  thus  obtain  an  in- 
finite number  of  perpendiculars,  of  gradually  increasing 
length,  as  D  approaches  F.  Uniting  the  ends  of  all 
these  perpendiculars,  we  obtain  a  curve,  and  between 
this  curve  and  the  straight  line  joining  F  and  D  we 
have  an  area  containing  all  the  perpendiculars  placed 
side  by  side.  Each  one  of  this  infinite  series  of  perpen- 
diculars representing  an  attraction,  or  tension,  as  it  is 
sometimes  called,  the  area  just  referred  to  represents 
the  sum  of  the  tensions  exerted  upon  the  particle  D,  dur- 
ing its  passage  from  its  first  position  to  F. 

Up  to  the  present  point  we  have  been  dealing  with 
tensions,  not  with  motion.  Thus  far  vis  viva  has  been 
entirely  foreign  to  our  contemplation  of  D  and  F.  Let 
us  now  suppose  D  placed  at  a  practically  infinite  distance 
from  F;  here,  as  stated,  the  pull  of  gravity  would  be 
infinitely  small,  and  the  perpendicular  representing  it 
would  dwindle  almost  to  a  point.  In  this  position  the 
sum  of  the  tensions  capable  of  being  exerted  on  D  would 
be  a  maximum.  Let  D  now  begin  to  move  in  obedience 
to  the  infinitesimal  attraction  exerted  upon  it.  Motion 
being  once  set  up,  the  idea  of  vis  viva  arises.  In  moving 
towards  F  the  particle  D  consumes,  as  it  were,  the  ten- 
sions. Let  us  fix  our  attention  on  D,  at  any  point  of 
the  path  over  which  it  is  moving.  Between  that  point 


THE    CONSTITUTION    OP    NATURE.  19 

and  F  there  is  a  quantity  of  unused  tensions;  beyond 
that  point  the  tensions  have  been  all  consumed,  but 
we  have  in  their  place  an  equivalent  quantity  of  vis 
viva.  After  D  has  passed  any  point,  the  tension  pre- 
viously in  store  at  that  point  disappears,  but  not  with- 
out having  added,  during  the  infinitely  small  duration 
of  its  action,  a  due  amount  of  motion  to  that  previously 
possessed  by  D.  The  nearer  D  approaches  to  F,  the 
smaller  is  the  sum  of  the  tensions  remaining,  but  the 
greater  is  the  vis  viva;  the  farther  D  is  from  F,  the 
greater  is  the  sum  of  the  unconsumed  tensions,  and 
the  less  is  the  living  force.  Now  the  principle  of  con- 
servation affirms  not  the  constancy  of  the  value  of  the 
tensions  of  gravity,  nor  yet  the  constancy  of  the  vis 
viva,  taken  separately,  but  the  absolute  constancy  of 
the  value  of  both  taken  together.  At  the  beginning 
the  vis  viva  was  zero,  and  the  tension  area  was  a  maxi- 
mum; close  to  F  the  vis  viva  is  a  maximum,  while  the 
tension  area  is  zero.  At  every  other  point,  the  work- 
producing  power  of  the  particle  D  consists  in  part  of 
vis  viva,  and  in  part  of  tensions. 

If  gravity,  instead  of  being  attraction,  were  repul- 
sion, then,  with  the  particles  in  contact,  the  sum  of  the 
tensions  between  D  and  F  would  be  a  maximum,  and  the 
vis  viva  zero.  If,  in  obedience  to  the  repulsion,  D 
moved  away  from  F,  vis  viva  would  be  generated;  and 
the  farther  D  retreated  from  F  the  greater  would  be  its 
vis  viva,  and  the  less  the  amount  of  tension  still  avail- 
able for  producing  motion.  Taking  repulsion  as  well 
as  attraction  into  account,  the  principle  of  the  conserva- 
tion of  force  affirms  that  the  mechanical  value  of  the 
tensions  and  vires  vivce  of  the  material  universe,  so  far 
as  we  know  it,  is  a  constant  quantity.  The  universe,  in 
short,  possesses  two  kinds  of  property  which  are  mutu- 
ally convertible.  The  diminution  of  either  carries  with 


20  FRAGMENTS    OF    SCIENCE. 

it  the  enhancement  of  the  other,  the  total  value  of  the 
property  remaining  unchanged. 

The  considerations  here  applied  to  gravity  apply 
equally  to  chemical  affinity.  In  a  mixture  of  oxygen 
and  hydrogen  the  atoms  exist  apart,  but  by  the  appli- 
cation of  proper  means  they  may  be  caused  to  rush  to- 
gether across  that  space  that  separates  them.  While 
this  space  exists,  and  as  long  as  the  atoms  have  not  be- 
gun to  move  towards  each  other,  we  have  tensions  and 
nothing  else.  During  their  motion  towards  each  other 
the  tensions,  as  in  the  case  of  gravity,  are  converted  into 
vis  viva.  After  they  clash  we  have  still  vis  viva,  but 
in  another  form.  It  was  translation,  it  is  vibration. 
It  was  molecular  transfer,  it  is  heat. 

It  is  possible  to  reverse  these  processes,  to  unlock 
the  combined  atoms  and  replace  them  in  their  first 
positions.  But,  to  accomplish  this,  as  much  heat  would 
be  required  as  was  generated  by  their  union.  Such  re- 
versals occur  daily  and  hourly  in  nature.  By  the  solar 
waves,  the  oxygen  of  water  is  divorced  from  its  hydrogen 
in  the  leaves  of  plants.  As  molecular  vis  viva  the 
waves  disappear,  but  in  so  doing  they  re-endow  the 
atoms  of  oxygen  and  hydrogen  with  tension.  The 
atoms  are  thus  enabled  to  recombine,  and  when  they 
do  so  they  restore  the  precise  amount  of  heat  consumed 
in  their  separation.  The  same  remarks  apply  to  the 
compound  of  carbon  and  oxygen,  called  carbonic  acid, 
which  is  exhaled  from  our  lungs,  produced  by  our  fires, 
and  found  sparingly  diffused  everywhere  throughout 
the  air.  In  the  leaves  of  plants  the  sunbeams  also 
wrench  the  atoms  of  carbonic  acid  asunder,  and  sacri- 
fice themselves  in  the  act;  but  when  the  plants  are 
burnt,  the  amount  of  heat  consumed  in  their  produc- 
,tion  is  restored. 

This,  then,  is  the  rhythmic  play  of  Nature  as  re- 


THE    CONSTITUTION    OF    NATURE.  21 

gards  her  forces.  Throughout  all  her  regions  she  oscil- 
lates from  tension  to  vis  viva,  from  vis  viva  to  tension. 
We  have  the  same  play  in  the  planetary  system.  The 
earth's  orbit  is  an  ellipse,  one  of  the  foci  of  which  is 
occupied  by  the  sun.  Imagine  the  earth  at  the  most 
distant  part  of  the  orbit.  Her  motion,  and  conse- 
quently her  vis  viva,  is  then  a  minimum.  The  planet 
rounds  the  curve,  and  begins  its  approach  to  the  sun. 
In  front  it  has  a  store  of  tensions,  which  are  gradu- 
ally consumed,  an  equivalent  amount  of  vis  viva 
being  generated.  When  nearest  to  the  sun  the  mo- 
tion, and  consequently  the  vis  viva,  reach  a  maxi- 
mum. But  here  the  available  tensions  have  been  used 
up.  The  earth  rounds  this  portion  of  the  curve  and 
retreats  from  the  sun.  Tensions  are  now  stored  up, 
but  vis  viva  is  lost,  to  be  again  restored  at  the  expense 
of  the  complementary  force  on  the  opposite  side  of 
the  curve.  Thus  beats  the  heart  of  the  universe,  but 
without  increase  or  diminution  of  its  total  stock  of 
force. 

I  have  thus  far  tried  to  steer  clear  amid  confusion, 
by  fixing  the  mind  of  the  reader  upon  things  rather 
than  upon  names.  But  good  names  are  essential;  and 
here,  as  yet,  we  are  not  provided  with  such.  We  have 
had  the  force  of  gravity  and  living  force — two  utterly 
distinct  things.  We  have  had  pulls  and  tensions;  and 
we  might  have  had  the  force  of  heat,  the  force  of  light, 
the  force  of  magnetism,  or  the  force  of  electricity — 
all  of  which  terms  have  been  employed  more  or  less 
loosely  by  writers  on  physics.  This  confusion  is  hap- 
pily avoided  by  the  introduction  of  the  term  '  energy/ 
which  embraces  both  tension  and  vis  viva.  Energy  is 
possessed  by  bodies  already  in  motion;  it  is  then  actual, 
and  we  agree  to  call  it  actual  or  dynamic  energy.  It 
is  our  old  vis  viva.  On  the  other  hand,  energy  is 


22  FKAGMENTS    OF    SCIENCE. 

possible  to  bodies  not  in  motion,  but  which,  in  virtue 
of  attraction  or  repulsion,  possess  a  power  of  motion 
which  would  realise  itself  if  all  hindrances  were  re- 
moved. Looking,  for'  example,  at  gravity;  a  body  on 
the  earth's  surface  in  a  position  from  which  it  cannot 
fall  to  a  lower  one  possesses  no  energy.  It  has  neither 
motion  nor  power  of  motion.  But  the  same  body  sus- 
pended at  a  height  above  the  earth  has  a  power  of  mo- 
tion, though  it  may  not  have  exercised  it.  Energy 
is  possible  to  such  a  body,  and  we  agree  to  call  this 
potential  energy.  It  consists  of  our  old  tensions.  We, 
moreover,  speak  of  the  conservation  of  energy,  instead 
of  the  conservation  of  force;  and  say  that  the  sum  of 
the  potential  and  dynamic  energies  of  the  material  uni- 
verse is  a  constant  quantity. 

A  body  cast  upwards  consumes  the  actual  energy  of 
projection,  and  lays  up  potential  energy.  When  it 
reaches  its  utmost  height  all  its  actual  energy  is  con- 
sumed, its  potential  energy  being  then  a  maximum. 
When  it  returns,  there  is  a  reconversion  of  the  poten- 
tial into  the  actual.  A  pendulum  at  the  limit  of  its 
swing  possesses  potential  energy;  at  the  lowest  point 
of  its  arc  its  energy  is  all  actual.  A  patch  of  snow 
resting  on  a  mountain  slope  has  potential  energy; 
loosened,  and  shooting  down  as  an  avalanche,  it  pos- 
sesses dynamic  energy.  The  pine-trees  growing  on  the 
Alps  have  potential  energy;  but  rushing  down  the 
Holzrinne  of  the  woodcutters  they  possess  actual  en- 
ergy. The  same  is  true  of  the  mountains  themselves. 
As  long  as  the  rocks  which  compose  them  can  fall  to  a 
lower  level,  they  possess  potential  energy,  which  is  con- 
verted into  actual  when  the  frost  ruptures  their  cohe- 
sion and  hands  them  over  to  the  action  of  gravity. 
The  stone  avalanches  of  the  Matterhorn  and  Weisshorn 
are  illustrations  in  point.  The  hammer  of  the  great 


THE    CONSTITUTION   OF   NATURE.  23 

bell  of  Westminster,  when  raised  before  striking,  pos- 
sesses potential  energy;  when  it  falls,  the  energy  be- 
comes dynamic;  and  after  the  stroke,  we  have  the 
rhythmic  play  of  potential  and  dynamic  in  the  vibra- 
tions of  the  bell.  The  same  holds  good  for  the  molecular 
oscillations  of  a  heated  body.  An  atom  is  driven  against 
its  neighbour,  and  recoils.  The  ultimate  amplitude  of 
the  recoil  being  attained,  the  motion  of  the  atom  in 
that  direction  is  checked,  and  for  an  instant  its  energy 
is  all  potential.  It  is  then  drawn  towards  its  neighbour 
with  accelerated  speed;  thus,  by  attraction,  converting 
its  potential  into  dynamic  energy.  Its  motion  in  this 
direction  is  also  finally  checked,  and  again,  for  an  in- 
stant, its  energy  is  all  potential.  It  once  more  retreats, 
converting,  by  repulsion,  its  potential  into  dynamic 
energy,  till  the  latter  attains  a  maximum,  after  which  it 
is  again  changed  into  potential  energy.  Thus,  what  is 
true  of  the  earth,  as  she  swings  to  and  fro  in  her  yearly 
journey  round  the  sun,  is  also  true  of  her  minutest 
atom.  We  have  wheels  within  wheels,  and  rhythm 
within  rhythm. 

When  a  body  is  heated,  a  change  of  molecular  ar- 
rangement always  occurs,  and  to  produce  this  change 
heat  is  consumed.  Hence,  a  portion  only  of  the  heat 
communicated  to  the  body  remains  as  dynamic  energy. 
Looking  back  on  some  of  the  statements  made  at  the 
beginning  of  this  article,  now  that  our  knowledge  is 
more  extensive,  we  see  the  necessity  of  qualifying  them. 
When,  for  example,  two  bodies  clash,  heat  is  generated; 
but  the  heat,  or  molecular  dynamic  energy,  developed 
at  the  moment  of  collision,  is  not  the  exact  equivalent 
of  the  sensible  dynamic  energy  destroyed.  The  true 
equivalent  is  this  heat,  plus  the  potential  energy  con- 
ferred upon  the  molecules  by  the  placing  of  greater 
distances  between  them.  This  molecular  potential  en- 
3 


24  FEAGMENTS    OF    SCIENCE. 

ergy  is  afterwards,  on  the  cooling  of  the  body,  con- 
verted into  heat. 

Wherever  two  atoms  capable  of  uniting  together  by 
their  mutual  attractions  exist  separately,  they  form  a 
store  of  potential  energy.  Thus  our  woods,  forests, 
and  coal-fields  on  the  one  hand,  and  our  atmospheric 
oxygen  on  the  other,  constitute  a  vast  store  of  energy  of 
this  kind — vast,  but  far  from  infinite.  We  have,  be- 
sides our  coal-fields,  metallic  bodies  more  or  less  sparsely 
distributed  through  the  earth's  crust.  These  bodies 
can  be  oxydised;  and  hence  they  are,  so  far  as  they  go, 
stores  of  energy.  But  the  attractions  of  the  great  mass 
of  the  earth's  crust  are  already  satisfied,  and  from  them 
no  further  energy  can  possibly  be  obtained.  Ages  ago 
the  elementary  constituents  of  our  rocks  clashed  to- 
gether and  produced  the  motion  of  heat,  which  was 
taken  up  by  the  ether  and  carried  away  through  stellar 
space.  It  is  lost  for  ever  as  far  as  we  are  concerned. 
In  those  ages  the  hot  conflict  of  carbon,  oxygen,  and  cal- 
cium produced  the  chalk  and  limestone  hills  which  are 
now  cold;  and  from  this  carbon,  oxygen,  and  cal- 
cium no  further  energy  can  be  derived.  So  it  is  with 
almost  all  the  other  constituents  of  the  earth's  crust. 
They  took  their  present  f  orm  in  obedience  to  molecular 
force;  they  turned  their  potential  energy  into  dynamic, 
and  yielded  it  as  radiant  heat  to  the  universe,  ages  be- 
fore man  appeared  upon  this  planet.  For  him  a  residue 
of  potential  energy  remains,  vast,  truly,  in  relation  to 
the  life  and  wants  of  an  individual,  but  exceedingly 
minute  in  comparison  with  the  earth's  primitive  store. 

To  sum  up.  The  whole  stock  of  energy  or  working- 
power  in  the  world  consists  of  attractions,  repiilsions, 
and  motions.  If  the  attractions  and  repulsions  be  so 
circumstanced  as  to  be  able  to  produce  motion,  they  are 
sources  of  working-power,  but  not  otherwise.  As  stated 


THE   CONSTITUTION   OF   NATURE.  25 

a  moment  ago,  the  attraction  exerted  between  the  earth 
and  a  body  at  a  distance  from  the  earth's  surface,  is  a 
source  of  working-power;  because  the  body  can  be 
moved  by  the  attraction,  and  in  falling  can  perform 
work.  When  it  rests  at  its  lowest  level  it  is  not  a  source 
of  power  or  energy,  because  it  can  fall  no  farther.  But 
though  it  has  ceased  to  be  a  source  of  energy,  the  at- 
traction of  gravity  still  acts  as  a  force,  which  holds  the 
earth  and  weight  together. 

The  same  remarks  apply  to  attracting  atoms  and 
molecules.  As  long  as  distance  separates  them,  they 
can  move  across  it  in  obedience  to  the  attraction;  and 
the  motion  thus  produced  may,  by  proper  appliances, 
be  caused  to  perform  mechanical  work.  When,  for  ex- 
ample, two  atoms  of  hydrogen  unite  with  one  of  oxygen, 
to  form  water,  the  atoms  are  first  drawn  towards  each 
other — they  move,  they  clash,  and  then  by  virtue  of 
their  resiliency,  they  recoil  and  quiver.  To  this  quiver- 
ing motion  we  give  the  name  of  heat.  This  atomic 
vibration  is  merely  the  redistribution  of  the  motion 
produced  by  the  chemical  affinity;  and  this  is  the  only 
sense  in  which  chemical  affinity  can  be  said  to  be  con- 
verted into  heat.  We  must  not  imagine  the  chemical 
attraction  destroyed,  or  converted  into  anything  else. 
For  the  atoms,  when  mutually  clasped  to  form  a  mole- 
cule of  water,  are  held  together  by  the  very  attraction 
which  first  drew  them  towards  each  other.  That  which 
has  really  been  expended  is  the  pull  exerted  through 
the  space  by  which  the  distance  between  the  atoms  has 
been  diminished. 

If  this  be  understood,  it  will  be  at  once  seen  that 
gravity,  as  before  insisted  on,  may,  in  this  sense,  be  said 
to  be  convertible  into  heat;  that  it  is  in  reality  no  more 
an  outstanding  and  inconvertible  agent,  as  it  is  some- 
times stated  to  be,  than  is  chemical  affinity.  By  the 


26  FRAGMENTS    OF    SCIENCE. 

exertion  of  a  certain  pull  through  a  certain  space,  a, 
body  is  caused  to  clash  with  a  certain  definite  velocity 
against  the  earth.  Heat  is  thereby  developed,  and  this 
is  the  only  sense  in  which  gravity  can  be  said  to  be  con- 
verted into  heat.  In  no  case  is  the  force  which  pro- 
duces the  motion  annihilated  or  changed  into  anything 
else.  The  mutual  attraction  of  the  earth  and  weight 
exists  when  they  are  in  contact,  as  when  they  were 
separate;  but  the  ability  of  that  attraction  to  employ 
itself  in  the  production  of  motion  does  not  exist. 

The  transformation,  in  this  case,  is  easily  followed 
by  the  mind's  eye.  First,  the  weight  as  a  whole  is  set 
in  motion  by  the  attraction  of  gravity.  This  motion  of 
the  mass  is  arrested  by  collision  with  the  earth,  being 
broken  up  into  molecular  tremors,  to  which  we  give  the 
name  of  heat. 

And  when  we  reverse  the  process,  and  employ  those 
tremors  of  heat  to  raise  a  weight — which  is  done 
through  the  intermediation  of  an  elastic  fluid  in  the 
steam-engine — a  certain  definite  portion  of  the  molecu- 
lar motion  is  consumed.  In  this  sense,  and  in  this  sense 
only,  can  the  heat  be  said  to  be  converted  into  gravity; 
or,  more  correctly,  into  potential  energy  of  gravity. 
Here  the  destruction  of  the  heat  has  created  no  new  at- 
traction; but  the  old  attraction  has  conferred  upon  it  a 
power  of  exerting  a  certain  definite  pull,  between  the 
starting-point  of  the  falling  weight  and  the  earth. 

When,  therefore,  writers  on  the  conservation  of  en- 
ergy speak  of  tensions  being  '  consumed '  and  '  gene- 
rated/ they  do  not  mean  thereby  that  old  attractions 
have  been  annihilated,  and  new  ones  brought  into  ex- 
istence, but  that,  in  the  one  case,  the  power  of  the  at- 
traction to  produce  motion  has  been  diminished  by  the 
shortening  of  the  distance  between  the  attracting 
bodies,  while,  in  the  other  case,  the  power  of  producing 


THE    CONSTITUTION    OP    NATURE.  27 

motion  has  been  augmented  by  the  increase  of  the  dis- 
tance. These  remarks  apply  to  all  bodies,  whether 
they  be  sensible  masses  or  molecules. 

Of  the  inner  quality  that  enables  matter  to  attract 
matter  we  know  nothing;  and  the  law  of  conservation 
makes  no  statement  regarding  that  quality.  It  takes 
the  facts  of  attraction  as  they  stand,  and  affirms  only 
the  constancy  of  working-power.  That  power  may  ex- 
ist in  the  form  of  MOTION;  or  it  may  exist  in  the  form  of 
FORCE,  with  distance  to  act  through.  The  former  is 
dynamic  energy,  the  latter  is  potential  energy,  the  con- 
stancy of  the  sum  of  both  being  affirmed  by  the  law  of 
conservation.  The  convertibility  of  natural  forces  con- 
sists solely  in  transformations  of  dynamic  into  poten- 
tial, and  of  potential  into  dynamic  energy.  In  no  other 
sense  has  the  convertibility  of  force  any  scientific  mean- 
ing. 


Grave  errors  have  been  entertained  as  to  what  is 
really  intended  to  be  conserved  by  the  doctrine  of  con- 
servation. This  exposition  I  hope  will  tend  to  remove 
them. 


II. 

RADIATION* 

1.  Visible  and  Invisible  Radiation. 

T3ETWEEN  the  mind  of  man  and  the  outer  world 
T*-'  are  interposed  the  nerves  of  the  human  body, 
which  translate,  or  enable  the  mind  to  translate,  the 
impressions  of  that  world  into  facts  of  consciousness 
and  thought. 

Different  nerves  are  suited  to  the  perception  of  dif- 
ferent impressions.  We  do  not  see  with  the  ear,  nor 
hear  with  the  eye,  nor  are  we  rendered  sensible  of  sound 
by  the  nerves  of  the  tongue.  Out  of  the  general  assem- 
blage of  physical  actions,  each  nerve,  or  group  of  nerves, 
selects  and  responds  to  those  for  the  perception  of 
which  it  is  especially  organised. 

The  optic  nerve  passes  from  the  brain  to  the  back 
of  the  eyeball  and  there  spreads  out,  to  form  the  retina, 
a  web  of  nerve  filaments,  on  which  the  images  of  ex- 
ternal objects  are  projected  by  the  optical  portion  of 
the  eye.  This  nerve  is  limited  to  the  apprehension  of 
the  phenomena  of  radiation,  and,  notwithstanding  its 
marvellous  sensibility  to  certain  impressions  of  this 
class,  it  is  singularly  obtuse  to  other  impressions. 

Nor  does  the  optic  nerve  embrace  the  entire  range 

*  The  Rede  Lecture  delivered  in  the  Senate  House  before  the 
University  of  Cambridge,  May  16,  1865. 
28 


RADIATION.  29 

even  of  radiation.  Some  rays,  when  they  reach  it,  are 
incompetent  to  evoke  its  power,  while  others  never 
reach  it  at  all,  being  absorbed  by  the  humours  of  the 
eye.  To  all  rays  which,  whether  they  reach  the  retina 
or  not,  fail  to  excite  vision,  we  give  the  name  of  invisible 
or  obscure  rays.  All  non-luminous  bodies  emit  such 
rays.  There  is  no  body  in  nature  absolutely  cold,  and 
every  body  not  absolutely  cold  emits  rays  of  heat.  But 
to  render  radiant  heat  fit  to  affect  the  optic  nerve  a 
certain  temperature  is  necessary.  A  cool  poker  thrust 
into  a  fire  remains  dark  for  a  time,  but  when  its  tem- 
perature has  become  equal  to  that  of  the  surrounding 
coals,  it  glows  like  them.  In  like  manner,  if  a  current 
of  electricity,  of  gradually  increasing  strength,  be  sent 
through  a  wire  of  the  refractory  metal  platinum,  the 
wire  first  becomes  sensibly  warm  to  the  touch;  for  a 
time  its  heat  augments,  still  however  remaining  ob- 
scure; at  length  we  can  no  longer  touch  the  metal  with 
impunity;  and  at  a  certain  definite  temperature  it  emits 
a  feeble  red  light.  As  the  current  augments  in  power 
the  light  augments  in  brilliancy,  until  finally  the  wire 
appears  of  a  dazzling  white.  The  light  which  it  now 
emits  is  similar  to  that  of  the  sun. 

By  means  of  a  prism  Sir  Isaac  Newton  unravelled 
the  texture  of  solar  light,  and  by  the  same  simple  instru- 
ment we  can  investigate  the  luminous  changes  of  our 
platinum  wire.  In  passing  through  the  prism  all  its 
rays  (and  they  are  infinite  in  variety)  are  bent  or  re- 
fracted from  their  straight  course;  and,  as  different 
rays  are  differently  refracted  by  the  prism,  we  are  by 
it  enabled  to  separate  one  class  of  rays  from  another. 
By  such  prismatic  analysis  Dr.  Draper  has  shown,  that 
when  the  platinum  wire  first  begins  to  glow,  the  light 
emitted  is  sensibly  red.  As  the  glow  augments  the  red 
becomes  more  brilliant,  but  at  the  same  time  orange  rays 


30  FRAGMENTS    OF    SCIENCE. 

are  added  to  the  emission.  Augmenting  the  tempera- 
ture still  further,  yellow  rays  appear  beside  the  orange; 
after  the  yellow,  green  rays  are  emitted;  and  after  the 
green  come,  in  succession,  blue,  indigo,  and  violet  rays. 
To  display  all  these  colours  at  the  same  time  the  platin- 
um wire  must  be  white-hot:  the  impression  of  white- 
ness being  in  fact  produced  by  the  simultaneous  action 
of  all  these  colours  on  the  optic  nerve. 

In  the  experiment  just  described  we  began  with  a 
platinum  wire  at  an  ordinary  temperature,  and  gradu- 
ally raised  it  to  a  white  heat.  At  the  beginning,  and 
even  before  the  electric  current  had  acted  at  all  upon 
the  wire,  it  emitted  invisible  rays.  For  some  time  after 
the  action  of  the  current  had  commenced,  and  even  for 
a  time  after  the  wire  had  become  intolerable  to  the 
touch,  its  radiation  was  still  invisible.  The  question 
now  arises,  What  becomes  of  these  invisible  rays  when 
the  visible  ones  make  their  appearance?  It  will 
be  proved  in  the  -sequel  that  they  maintain  them- 
selves in  the  radiation;  that  a  ray  once  emitted  contin- 
ues to  be  emitted  when  the  temperature  is  increased, 
and  hence  the  emission  from  our  platinum  wire,  even 
when  it  has  attained  its  maximum  brilliancy,  consists 
of  a  mixture  of  visible  and  invisible  rays.  If,  instead  of 
the  platinum  wire,  the  earth  itself  were  raised  to  incan- 
descence, the  obscure  radiation  which  it  now  emits 
would  continue  to  be  emitted.  To  reach  incandescence 
the  planet  would  have  to  pass  through  all  the  stages  of 
non-luminous  radiation,  and  the  final  emission  would 
embrace  the  rays  of  all  these  stages.  There  can  hardly 
be  a  doubt  that  from  the  sun  itself,  rays  proceed  similar 
in  kind  to  those  which  the  dark  earth  pours  nightly  into 
space.  In  fact,  the  various  kind  of  obscure  rays  emitted 
by  all  the  planets  of  our  system  are  included  in  the 
present  radiation  of  the  sun. 


RADIATION.  31 

The  great  pioneer  in  this  domain  of  science  was  Sir 
William  Herschel.  Causing  a  beam  of  solar  light  to 
pass  through  a  prism,  he  resolved  it  into  its  coloured 
constituents;  he  formed  what  is  technically  called  the 
solar  spectrum.  Exposing  thermometers  to  the  suc- 
cessive colours  he  determined  their  heating  power,  and 
found  it  to  augment  from  the  violet  or  most  refracted 
end,  to  the  red  or  least  refracted  end  of  the  spectrum. 
But  he  did  not  stop  here.  Pushing  his  thermometers 
into  the  dark  space  beyond  the  red  he  found  that, 
though  the  light  had  disappeared,  the  radiant  heat 
falling  on  the  instruments  was  more  intense  than  that 
at  any  visible  part  of  the  spectrum.  In  fact,  Sir 
William  Herschel  showed,  and  his  results  have  been 
verified  by  various  philosophers  since  his  time,  that,  be- 
sides its  luminous  rays,  the  sun  pours  forth  a  multitude 
of  other  rays,  more  powerfully  calorific  than  the  lumin- 
ous ones,  but  entirely  unsuited  to  the  purposes  of  vision. 

At  the  less  refrangible  end  of  the  solar  spectrum, 
then,  the  range  of  the  sun's  radiation  is  not  limited  by 
that  of  the  eye.  The  same  statement  applies  to  the 
more  refrangible  end.  Hitter  discovered  the  extension 
of  the  spectrum  into  the  invisible  region  beyond  the 
violet;  and,  in  recent  times,  this  ultra-violet  emission 
has  had  peculiar  interest  conferred  upon  it  by  the  ad- 
mirable researches  of  Professor  Stokes.  The  complete 
spectrum  of  the  sun  consists,  therefore,  of  three  distinct 
parts: — first,  of  ultra-red  rays  of  high  heating  power, 
but  unsuited  to  the  purposes  of  vision;  secondly,  of 
luminous  rays  which  display  the  succession  of  colours, 
red,  orange,  yellow,  green,  blue,  indigo,  violet;  thirdly, 
of  ultra-violet  rays  which,  like  the  ultra-red  ones,  are 
incompetent  to  excite  vision,  but  which,  unlike  the 
ultra-red  rays,  possess  a  very  feeble  heating  power.  In 
consequence,  however,  of  their  chemical  energy  these 


32  FRAGMENTS    OF    SCIENCE. 

ultra-violet  rays  are  of  the  utmost  importance  to  the 
organic  world. 


2.  Origin  and  Character  of  Radiation.     The  Ether. 

When  we  see  a  platinum  wire  raised  gradually  to  a 
white  heat,  and  emitting  in  succession  all  the  colours  of 
the  spectrum,  we  are  simply  conscious  of  a  series  of 
changes  in  the  condition  of  our  own  eyes.  We  do  not 
see  the  actions  in  which  these  successive  colours  origin- 
ate, but  the  mind  irresistibly  infers  that  the  appearance 
of  the  colours  corresponds  to  certain  contemporaneous 
changes  in  the  wire.  What  is  the  nature  of  these 
changes?  In  virtue  of  what  condition  does  the  wire 
radiate  at  all?  We  must  now  look  from  the  wire,  as 
a  whole,  to  its  constituent  atoms.  Could  we  see  those 
atoms,  even  before  the  electric  current  has  begun  to 
act  upon  them,  we  should  find  them  in  a  state  of  vibra- 
tion. In  this  vibration,  indeed,  consists  such  warmth 
as  the  wire  then  possesses.  Locke  enunciated  this  idea 
with  great  precision,  and  it  has  been  placed  beyond  the 
pale  of  doubt  by  the  excellent  quantitative  researches  of 
Mr.  Joule.  '  Heat/  says  Locke,  '  is  a  very  brisk  agita- 
tion of  the  insensible  parts  of  the  object,  which  produce 
in  us  that  sensation  from  which  we  denominate  the  ob- 
ject hot:  so  what  in  our  sensations  is  heat  in  the  object 
is  nothing  but  motion.'  When  the  electric  current, 
still  feeble,  begins  to  pass  through  the  wire,  its  first 
act  is  to  intensify  the  vibrations  already  existing,  by 
causing  the  atoms  to  swing  through  wider  ranges.  Tech- 
nically speaking,  the  amplitudes  of  the  oscillations  are 
increased.  The  current  does  this,  however,  without 
altering  the  periods  of  the  old  vibrations,  or  the  times 
in  which  they  were  executed.  But  besides  intensifying 


RADIATION.  33 

the  old  vibrations  the  current  generates  new  and  more 
rapid  ones,  and  when  a  certain  definite  rapidity  has  been 
attained,  the  wire  begins  to  glow.  The  colour  first 
exhibited  is  red,  which  corresponds  to  the  lowest  rate 
of  vibration  of  which  the  eye  is  able  to  take  cognisance. 
By  augmenting  the  strength  of  the  electric  current 
more  rapid  vibrations  are  introduced,  and  orange  rays 
appear.  A  quicker  rate  of  vibration  produces  yellow,  a 
still  quicker  green;  and  by  further  augmenting  the 
rapidity,  we  pass  through  blue,  indigo,  and  violet,  to 
the  extreme  ultra-violet  rays. 

Such  are  the  changes  recognised  by  the  mind  in 
the  wire  itself,  as  concurrent  with  the  visual  changes 
taking  place  in  the  eye.  But  what  connects  the  wire 
with  this  organ?  By  what  means  does  it  send  such  in- 
telligence of  its  varying  condition  to  the  optic  nerve? 
Heat  being  as  denned  by  Locke, '  a  very  brisk  agitation 
of  the  insensible  parts  of  an  object/  it  is  readily  con- 
ceivable that  on  touching  a  heated  body  the  agitation 
may  communicate  itself  to  the  adjacent  nerves,  and  an- 
nounce itself  to  them  as  light  or  heat.  But  the  optic 
nerve  does  not  touch  the  hot  platinum,  and  hence  the 
pertinence  of  the  question,  By  what  agency  are  the  vi- 
brations of  the  wire  transmitted  to  the  eye? 

The  answer  to  this  question  involves  one  of  the  most 
important  physical  conceptions  that  the  mind  of  man 
has  yet  achieved:  the  conception  of  a  medium  filling 
space  and  fitted  mechanically  for  the  transmission  of 
the  vibrations  of  light  and  heat,  as  air  is  fitted  for  the 
transmission  of  sound.  This  medium  is  called  the 
luminiferou's  ether.  Every  vibration  of  every  atom  of 
our  platinum  wire  raises  in  this  ether  a  wave,  which 
speeds  through  it  at  the  rate  of  186,000  miles  a  second. 
The  ether  suffers  no  rupture  of  continuity  at  the  sur- 
face of  the  eye,  the  inter-molecular  spaces  of  the  various 


34  FRAGMENTS    OF    SCIENCE. 

humours  are  filled  with  it;  hence  the  waves  generated 
by  the  glowing  platinum  can  cross  these  humours  and 
impinge  on  the  optic  nerve  at  the  back  of  the  eye.* 
Thus  the  sensation  of  light  reduces  itself  to  the  ac- 
ceptance of  motion.  Up  to  this  point  we  deal  with 
pure  mechanics;  but  the  subsequent  translation  of  the 
shock  of  the  ethereal  waves  into  consciousness  eludes 
mechanical  science.  As  an  oar  dipping  into  the  Cam 
generates  systems  of  waves,  which,  speeding  from  the 
centre  of  disturbance,  finally  stir  the  sedges  on  the 
river's  bank,  so  do  the  vibrating  atoms  generate  in  the 
surrounding  ether  undulations,  which  finally  stir  the 
filaments  of  the  retina.  The  motion  thus  imparted 
is  transmitted  with  measurable,  and  not  very  great 
velocity  to  the  brain,  where,  by  a  process  which  the 
science  of  mechanics  does  not  even  tend  to  unravel,  the 
tremor  of  the  nervous  matter  is  converted  into  the  con- 
scious impression  of  light. 

Darkness  might  then  be  defined  as  ether  at  rest; 
light  as  ether  in  motion.  But  in  reality  the  ether  is 
never  at  rest,  for  in  the  absence  of  light-waves  we  have 
heat-waves  always  speeding  through  it.  In  the  spaces 
of  the  universe  both  classes  of  undulations  incessantly 
commingle.  Here  the  waves  issuing  from  uncounted 
centres  cross,  coincide,  oppose,  and  pass  through  each 
other,  without  confusion  or  ultimate  extinction.  Every 
star  is  seen  across  the  entanglement  of  wave-motions 
produced  by  all  other  stars.  It  is  the  ceaseless  thrill 
caused  by  those  distant  orbs  collectively  in  the  ether, 
tha.t  constitutes  what  we  call  the  *  temperature  of 
space.'  As  the  air  of  a  room  accommodates'itself  to  the 
requirements  of  an  orchestra,  transmitting  each  vibra- 

*  The  action  here  described  is  analogous  to  the  passage  of 
sound-waves  through  thick  felt  whose  interstices  are  occupied 
by  air. 


RADIATION.  35 

tion  of  every  pipe  and  string,  so  does  the  inter-stellar 
ether  accommodate  itself  to  the  requirement  of  light  and 
heat.  Its  waves  mingle  in  space  without  disorder,  each 
being  endowed  with  an  individuality  as  indestructi- 
ble as  if  it  alone  had  disturbed  the  universal  re- 
pose. 

All  vagueness  with  regard  to  the  use  of  the  terms 
'  radiation '  and  '  absorption  '  will  now  disappear.  Ra- 
diation is  the  communication  of  vibratory  motion  to 
the  ether;  and  when  a  body  is  said  to  be  chilled  by 
radiation,  as  for  example  the  grass  of  a  meadow  on  a 
starlight  night,  the  meaning  is,  that  the  molecules  of 
the  grass  have  lost  a  portion  of  their  motion,  by  im- 
parting it  to  the  medium  in  which  they  vibrate.  On 
the  other  hand,  the  waves  of  ether  may  so  strike  against 
the  molecules  of  a  body  exposed  to  their  action  as  to 
yield  up  their  motion  to  the  latter;  and  in  this  transfer 
of  the  motion  from  the  ether  to  the  molecules  consists 
the  absorption  of  radiant  heat.  All  the  phenomena 
of  heat  are  in  this  way  reducible  to  interchanges  of  mo- 
tion; and  it  is  purely  as  the  recipients  or  the  donors  of 
this  motion,  that  we  ourselves  become  conscious  of  the 
action  of  heat  and  cold. 


3.  The  Atomic  Theory  in  reference  to  the  Ether. 

The  word  *  atoms '  has  been  more  than  once  em- 
ployed in  this  discourse.  Chemists  have  taught  us  that 
all  matter  is  reducible  to  certain  elementary  forms  to 
which  they  give  this  name.  These  atoms  are  endowed 
with  powers  of  mutual  attraction,  and  under  suitable 
circumstances  they  coalesce  to  form  compounds.  Thus 
oxygen  and  hydrogen  are  elements  when  separate,  or 
merely  mixed,  but  they  may  be  made  to  combine  so  as 


36  FRAGMENTS    OF   SCIENCE. 

to  form  molecules,  each  consisting  of  two  atoms  of 
hydrogen  and  one  of  oxygen.  In  this  condition  they 
constitute  water.  So  also  chlorine  and  sodium  are  ele- 
ments, the  former  a  pungent  gas,  the  latter  a  soft  metal; 
and  they  unite  together  to  form  chloride  of  sodium 
or  common  salt.  In  the  same  way  the  element  nitro- 
gen combines  with  hydrogen,  in  the  proportion  of  one 
atom  of  the  former  to  three  of  the  latter,  to  form  am- 
monia. Picturing  in  imagination  the  atoms  of  ele- 
mentary bodies  as  little  spheres,  the  molecules  of  com- 
pound bodies  must  be  pictured  as  groups  of  such 
spheres.  This  is  the  atomic  theory  as  Dalton  conceived 
it.  Now  if  this  theory  have  any  foundation  in  fact, 
and  if  the  theory  of  an  ether  pervading  space,  and  con- 
stituting the  vehicle  of  atomic  motion,  be  founded  in 
fact,  it  is  surely  of  interest  to  examine  whether  the  vi- 
brations of  elementary  bodies  are  modified  by  the  act 
of  combination — whether  as  regards  radiation  and  ab- 
sorption, or,  in  other  words,  whether  as  regards  the 
communication  of  motion  to  the  ether,  and  the  accept- 
ance of  motion  from  it,  the  deportment  of  the  uncom- 
bined  atoms  will  be  different  from  that  of  the  combined. 


4.  Absorption  of  Radiant  Heat  ~by  Gases. 

We  have  now  to  submit  these  considerations  to  the 
only  test  by  which  they  can  be  tried,  namely,  that  of 
experiment.  An  experiment  is  well  defined  as  a  ques- 
tion put  to  Nature;  but,  to  avoid  the  risk  of  asking 
amiss,  we  ought  to  purify  the  question  from  all  adjuncts 
which  do  not  necessarily  belong  to  it.  Matter  has 
been  shown  to  be  composed  of  elementary  constituents, 
by  the  compounding  of  which  all  its  varieties  are  pro- 
duced. But,  besides  the  chemical  unions  which  they 


RADIATION.  37 

form,  both  elementary  and  compound  bodies  can  unite 
in  another  and  less  intimate  way.  Gases  and  vapours 
aggregate  to  liquids  and  solids,  without  any  change  of 
their  chemical  nature.  We  do  not  yet  know  how  the 
transmission  of  radiant  heat  may  be  affected  by  the  en- 
tanglement due  to  cohesion;  and,  as  our  object  now  is 
to  examine  the  influence  of  chemical  union  alone,  we 
shall  render  our  experiments  more  pure  by  liberating 
the  atoms  and  molecules  entirely  from  the  bonds  of 
cohesion,  and  employing  them  in  the  gaseous  or  vapor- 
ous form. 

Let  us  endeavour  to  obtain  a  perfectly  clear  mental 
image  of  the  problem  now  before  us.  Limiting  in  the 
first  place  our  enquiries  to  the  phenomena  of  absorp- 
tion, we  have  to  picture  a  succession  of  waves  issuing 
from  a  radiant  source  and  passing  through  a  gas;  some 
of  them  striking  against  the  gaseous  molecules  and 
yielding  up  their  motion  to  the  latter;  others  gliding 
round  the  molecules,  or  passing  through  the  inter- 
molecular  spaces  without  apparent  hindrance.  The 
problem  before  us  is  to  determine  whether  such  free 
molecules  have^any  power  whatever  to  stop  the  waves 
of  heat;  and  if  so,  whether  different  molecules  possess 
this  power  in  different  degrees. 

In  examining  the  problem  let  us  fall  back  upon  an 
actual  piece  of  work,  choosing  as  the  source  of  our 
heat-waves  a  plate  of  copper,  against  the  back  of  which 
a  steady  sheet  of  flame  is  permitted  to  play.  On  emerg- 
ing from  the  copper,  the  waves,  in  the  first  instance, 
pass  through  a  space  devoid  of  air,  and  then  enter  a 
hollow  glass  cylinder,  three  feet  long  and  three  inches 
wide.  The  two  ends  of  this  cylinder  are  stopped  by 
two  plates  of  rock-salt,  a  solid  substance  which  offers  a 
scarcely  sensible  obstacle  to  the  passage  of  the  calorific 
waves.  After  passing  through  the  tube,  the  radiant 


38  FEAGMENTS    OF    SCIENCE. 

heat  falls  upon  the  anterior  face  of  a  thermo-electric 
pile,*  which  instantly  converts  the  heat  into  an  electric 
current.  This  current  conducted  round  a  magnetic 
needle  deflects  it,  and  the  magnitude  of  the  deflection 
is  a  measure  of  the  heat  falling  upon  the  pile.  This 
famous  instrument,  and  not  an  ordinary  thermometer, 
is  what  we  shall  use  in  these  enquiries,  but  we  shall  use 
it  in  a  somewhat  novel  way.  As  long  as  the  two  oppo- 
site faces  of  the  thermo-electric  pile  are  kept  at  the 
same  temperature,  no  matter  how  high  that  may  be, 
there  is  no  current  generated.  The  current  is  a  conse- 
quence of  a  difference  of  temperature  between  the  two 
opposite  faces  of  the  pile.  Hence,  if  after  the  anterior 
face  has  received  the  heat  from  our  radiating  source,  a 
second  source,  which  we  may  call  the  compensating 
source,  be  permitted  to  radiate  against  the  posterior 
face,  this  latter  radiation  will  tend  to  neutralise  the 
former.  When  the  neutralisation  is  perfect,  the  mag- 
netic needle  connected  with  the  pile  is  no  longer  de- 
flected, but  points  to  the  zero  of  the  graduated  circle 
over  which  it  hangs. 

And  now  let  us  suppose  the  glass^  tube,  through 
which  the  waves  from  the  heated  plate  of  copper  are 
passing,  to  be  exhausted  by  an  air-pump,  the  two 
sources  of  heat  acting  at  the  same  time  on  the  two 
opposite  faces  of  the  pile.  When  by  means  of  an  ad- 
justing screen,  perfectly  equal  quantities  of  heat  are 
imparted  to  the  two  faces,  the  needle  points  to  zero. 
Let  any  gas  be  now  permitted  to  enter  the  exhausted 
tube;  if  its  molecules  possess  any  power  of  intercepting 
the  calorific  waves,  the  equilibrium  previously  existing 
will  be  destroyed,  the  compensating  source  will  tri- 

*  In  the  Appendix  to  the  first  chapter  of  '  Heat  as  a  Mode  of 
Motion,'  the  construction  of  the  thermo-electric  pile  is  fully  ex- 
plained. 


RADIATION.  39 

umph,  and  a  deflection  of  the  magnetic  needle  will  be 
the  immediate  consequence.  From  the  deflections  thus 
produced  by  different  gases,  we  can  readily  deduce  the 
relative  amounts  of  wave-motion  which  their  molecules 
intercept. 

In  this  way  the  substances  mentioned  in  the  follow- 
ing table  were  examined,  a  small  portion  only  of  each 
being  admitted  into  the  glass  tube.  The  quantity  ad- 
mitted in  each  case  was  just  sufficient  to  depress  a  col- 
umn of  mercury  associated  with  the  tube  one  inch:  in 
other  words,  the  gases  were  examined  at  a  pressure  of 
one-thirtieth  of  an  atmosphere.  The  numbers  in  the 
table  express  the  relative  amounts  of  wave-motion  ab- 
sorbed by  the  respective  gases,  the  quantity  intercepted 
by  atmospheric  air  being  taken  as  unity. 

Radiation  through  Oases. 

Relative 
Name  of  gas  absorption 

Air 1 

Oxygen 1 

Nitrogen 1 

Hydrogen 1 

Carbonic  oxido .      '.     .  ."       .  v    .  •     .    . ".     750 

Carbonic  acid 972 

Hydrochloric  acid    .        ,      ?.....  .        .        .1,005 

Nitric  oxide      .       .       .       .       .        .        .  1,590 

Nitrous  oxide    .       .       .      :.       .        .        .1.860 

Sulphide  of  hydrogen 2,100 

Ammonia.  .  »  ,.  .-  .  .  .  5,460 
Olefiantgas  .  '  .  »  .  ,.-;  .  .  .  .  6,030. 
Sulphurous  acid  .  •  .•  .  •  •  .6,480 

Every  gas  in  this  table  is  perfectly  transparent  to 
light,  that  is  to  say,  all  waves  within  the  limits  of  the 
visible  spectrum  pass  through  it  without  obstruction; 
but  for  the  waves  of  slower  periods,  emanating  from  our 
heated  plate  of  copper,  enormous  differences  of  absorp- 
tive power  are  manifested.  These  differences  illustrate 
4 


40  FRAGMENTS    OF    SCIENCE. 

in  the  most  unexpected  manner  the  influence  of  chemi- 
cal combination.  Thus  the  elementary  gases,  oxygen, 
hydrogen,  and  nitrogen,  and  the  mixture  atmospheric 
air,  prove  to  be  practical  vacua  to  the  rays  of  heat;  for 
every  ray,  or,  more  strictly  speaking,  for  every  unit  of 
wave-motion,  which  any  one  of  them  intercepts,  per- 
fectly transparent  ammonia  intercepts  5,460  units, 
olefiant  gas  6,030  units,  while  sulphurous  acid  gas  ab- 
sorbs 6,480  units.  What  becomes  of  the  wave-motion 
thus  intercepted?  It  is  applied  to  the  heating  of  the 
absorbing  gas.  Through  air,  oxygen,  hydrogen,  and 
nitrogen,  the  waves  of  ether  pass  without  absorption, 
and  these  gases  are  not  sensibly  changed  in  tempera- 
ture by  the  most  powerful  calorific  rays.  The  position 
of  nitrous  oxide  in  the  foregoing  table  is  worthy  of 
particular  notice.  In  this  gas  we  have  the  same  atoms 
in  a  state  of  chemical  union,  that  exist  uncombined  in 
the  atmosphere;  but  the  absorption  of  the  compound  is 
1,800  times  that  of  air. 


5.  Formation  of  Invisible  Foci. 

This  extraordinary  deportment  of  the  elementary 
gases  naturally  directed  attention  to  elementary  bodies 
in  other  states  of  aggregation.  Some  of  Melloni's  re- 
sults now  attained  a  new  significance.  This  celebrated 
experimenter  had  found  crystals  of  sulphur  to  be  highly 
pervious  to  radiant  heat;  he  had  also  proved  that  lamp- 
black, and  black  glass,  (which  owes  its  blackness  to  the 
element  carbon)  were  to  a  considerable  extent  trans- 
parent to  calorific  rays  of  low  refrangibility.  These 
facts,  harmonising  so  strikingly  with  the  deportment  of 
the  simple  gases,  suggested  further  enquiry.  Sulphur 
dissolved  in  bisulphide  of  carbon  was  found  almost  per- 


RADIATION.  41 

fectly  diathermic.  The  dense  and  deeply-coloured  ele- 
ment bromine  was  examined,  and  found  competent  to 
cut  off  the  light  of  our  most  brilliant  flames,  while  it 
transmitted  the  invisible  calorific  rays  with  extreme 
freedom.  Iodine,  the  companion  element  of  bromine, 
was  next  thought  of,  but  it  was  found  impracticable  to 
examine  the  substance  in  its  usual  solid  condition.  It 
however  dissolves  freely  in  bisulphide  of  carbon.  There 
is  no  chemical  union  between  the  liquid  and  the  iodine; 
it  is  simply  a  case  of  solution,  in  which  the  uncombined 
atoms  of  the  element  can  act  upon  the  radiant  heat. 
When  permitted  to  do  so,  it  was  found  that  a  layer  of 
dissolved  iodine,  sufficiently  opaque  to  cut  off  the  light 
of  the  midday  sun,  was  almost  absolutely  transparent 
to  the  invisible  calorific  rays.* 

By  prismatic  analysis  Sir  "William  Herschel  sepa- 
rated the  luminous  from  the  non-luminous  rays  of  the 
sun,  and  he  also  sought  to  render  the  obscure  rays  visi- 
ble by  concentration.  Intercepting  the  luminous  por- 
tion of  his  spectrum  he  brought,  by  a  converging  lens, 
the  ultra-red  rays  to  a  focus,  but  by  this  condensation 
he  obtained  no  light.  The  solution  of  iodine  offers  a 
means  of  filtering  the  solar  beam,  or  failing  it,  the 
beam  of  the  electric  lamp,  which  renders  attainable  far 
more  powerful  foci  of  invisible  rays  than  could  possibly 
be  obtained  by  the  method  of  Sir  William  Herschel. 
For  to  form  his  spectrum  he  was  obliged  to  operate 
upon  solar  light  which  had  passed  through  a  narrow  slit 
or  through  a  small  aperture,  the  amount  of  the  obscure 
heat  being  limited  by  this  circumstance.  But  with  our 
opaque  solution  we  may  employ  the  entire  surface  of 
the  largest  lens,  and  having  thus  converged  the  rays, 

*  Professor  Dewnr  has  recently  succeeded  in  producing  a 
medium  highly  opaque  to  light,  and  highly  transparent  to  ob- 
scure heat,  by  fusing  together  sulphur  and  iodine. 


42  FRAGMENTS    OF    SCIENCE. 

luminous  and  non-luminous,  we  can  intercept  the  for- 
mer by  the  iodine,  and  do  what  we  please  with  the  lat- 
ter. Experiments  of  this  character,  not  only  with  the 
iodine  solution,  but  also  with  black  glass  and  layers  of 
lamp-black,  were  publicly  performed  at  the  Eoyal  Insti- 
tution in  the  early  part  of  18G2,  and  the  effects  at  the 
foci  of  invisible  rays,  then  obtained,  were  such  as  had 
never  been  witnessed  previously. 

In  the  experiments  here  referred  to,  glass  lenses 
were  employed  to  concentrate  the  rays.  But  glass, 
though  highly  transparent  to  the  luminous,  is  in  a  high 
degree  opaque  to  the  invisible,  heat-rays  of  the  electric 
lamp,  and  hence  a  large  portion  of  those  rays  was  in- 
tercepted by  the  glass.  The  obvious  remedy  here  is  to 
employ  rock-salt  lenses  instead  of  glass  ones,  or  to  aban- 
don the  use  of  lenses  wholly,  and  to  concentrate  the 
rays  by  a  metallic  mirror.  Both  of  these  improvements 
have  been  introduced,  and,  as  anticipated,  the  invisible 
foci  have  been  thereby  rendered  more  intense."  The 
mode  of  operating  remains  however  the  same,  in  prin- 
ciple, as  that  made  known  in  1862.  It  was  then  found 
that  an  instant's  exposure  of  the  face  of  the  thermo- 
electric pile  to  the  focus  of  invisible  rays,  dashed  the 
needles  of  a  coarse  galvanometer  violently  aside.  It 
is  now  found  that  on  substituting  for  the  face  of  the 
thermo-electric  pile  a  combustible  body,  the  invisible 
rays  are  competent  to  set  that  body  on  fire. 


6.  Visible  and  Invisible  Rays  of  the  Electric  Light. 

We  have  next  to  examine  what  proportion  the  non- 
luminous  rays  of  the  electric  light  bear  to  the  luminous 
ones.  This  the  opaque  solution  of  iodine  enables  us  to 
do  with  an  extremely  close  approximation  to  the  truth. 


RADIATION.  43 

The  pure  bisulphide  of  carbon,  which  is  the  solvent 
of  the  iodine,  is  perfectly  transparent  to  the  luminous, 
and  almost  perfectly  transparent  to  the  dark,  rays  of  the 
electric  lamp.  Supposing  the  total  radiation  of  the 
lamp  to  pass  through  the  transparent  bisulphide,  while 
through  the  solution  of  iodine  only  the  dark  rays  are 
transmitted.  If  we  determine,  by  means  of  a  thermo- 
electric pile,  the  total  radiation,  and  deduct  from  it 
the  purely  obscure,  we  obtain  the  value  of  the  purely 
luminous  emission.  Experiments,  performed  in  this 
way,  prove  that  if  all  the  visible  rays  of  the  electric  light 
were  converged  to  a  focus  of  dazzling  brilliancy,  its 
heat  would  only  be  one-eighth  of  that  produced  at  the 
unseen  focus  of  the  invisible  rays. 

Exposing  his  thermometers  to  the  successive  col- 
ours of  the  solar  spectrum,  Sir  William  Herschel  deter- 
mined the  heating  power  of  each,  and  also  that  of  the 
region  beyond  the  extreme  red.  Then  drawing  a 
straight  line  to  represent  the  length  of  the  spectrum,  he 
erected,  at  various  points,  perpendiculars  to  represent 
the  calorific  intensity  existing  at  those  points.  Uniting 
the  ends  of  all  his  perpendiculars,  he  obtained  a  curve 
which  showed  at  a  glance  the  manner  in  which  the 
heat  was  distributed  in  the  solar  spectrum.  Professor 
Miiller  of  Freiburg,  with  improved  instruments,  after- 
wards made  similar  experiments,  and  constructed  a 
more  accurate  diagram  of  the  same  kind.  We  have  now 
to  examine  the  distribution  of  heat  in  the  spectrum  of 
the  electric  light;  and  for  this  purpose  we  shall  employ 
a  particular  form  of  the  thermo-electric  pile,  devised  by 
Melloni.  Its  face  is  a  rectangle,  which  by  means  of 
movable  side-pieces  can  be  rendered  as  narrow  as  de- 
sired. We  can,  for  example,  have  the  face  of  the  pile 
the  tenth,  the  hundredth,  or  even  the  thousandth  of  an 
inch  in  breadth.  By  means  of  an  endless  screw,  this 


44  FRAGMENTS    OF    SCIENCE. 

linear  thermo-electric  pile  may  be  moved  through  the 
entire  spectrum,  from  the  violet  to  the  red,  the  amount 
of  heat  falling  upon  the  pile  at  every  point  of  its  march, 
being  declared  by  a  magnetic  needle  associated  with 
the  pile. 

When  this  instrument  is  brought  up  to  the  violet 
end  of  the  spectrum  of  the  electric  light,  the  heat  is 
found  to  be  insensible.  As  the  pile  is  gradually  moved 
from  the  violet  end  towards  the  red,  heat  soon  mani- 
fests itself,  augmenting  as  we  approach  the  red.  Of  all 
the  colours  of  the  visible  spectrum  the  red  possesses  the 
highest  heating  power.  On  pushing  the  pile  into  the 
dark  region  beyond  the  red,  the  heat,  instead  of  vanish- 
ing, rises  suddenly  and  enormously  in  intensity,  until 
at  some  distance  beyond  the  red  it  attains  a  maximum. 
Moving  the  pile  still  forward,  the  thermal  power  falls, 
somewhat  more  rapidly  than  it  rose.  It  then  gradually 
shades  away,  but,  for  a  distance  beyond  the  red  greater 
than  the  length  of  the  whole  visible  spectrum,  signs  of 
heat  may  be  detected. 

Drawing  a  datum  line,  and  erecting  along  it  per- 
pendiculars, proportional  in  length  to  the  thermal  in- 
tensity at  the  respective  points,  WTC  obtain  the  extraor- 
dinary curve,  shown  on  the  opposite  page,  which  ex- 
hibits the  distribution  of  heat  in  the  spectrum  of  the 
electric  light.  In  the  region  of  dark  rays,  beyond  the 
red,  the  curve  shoots  up  to  B,  in  a  steep  and  massive 
peak — a  kind  of  Matterhorn  of  heat,  which  dwarfs  the 
portion  of  the  diagram  c  D  E,  representing  the  luminous 
radiation.  Indeed  the  idea  forced  upon  the  mind  by 
this  diagram  is  that  the  light  rays  are  a  mere  insigni- 
ficant appendage  to  the  heat-rays  represented  by  the 
area  A  B  c  D,  thrown  in  as  it  were  by  nature  for  the  pur- 
pose of  vision. 

The  diagram  drawn  by  Professor  Miiller  to  repre- 


.RADIATION. 


46 


46  FRAGMENTS    OF    SCIENCE. 

sent  the  distribution  of  heat  in  the  solar  spectrum  is 
not  by  any  means  so  striking  as  that  just  described,  and 
the  reason,  doubtless,  is  that  prior  to  reaching  the  earth 
the  solar  rays  have  to  traverse  our  atmosphere.  By  the 
aqueous  vapour  there  diffused,  the  summit  of  the  peak 
representing  the  sun's  invisible  radiation  is  cut  off.  A 
similar  lowering  of  the  mountain  of  invisible  heat  is 
observed  when  the  rays  from  the  electric  light  are  per- 
mitted to  pass  through  a  film  of  water,  which  acts  upon 
them  as  the  atmospheric  vapour  acts  upon  the  rays  of 
the  sun. 


7.  Combustion  by  Invisible  Rays. 

The  sun's  invisible  rays  far  transcend  the  visible 
ones  in  heating  power,  so  that  if  the  alleged  perfor- 
mances of  Archimedes  during  the  siege  of  Syracuse  had 
any  foundation  in  fact,  the  dark  solar  rays  would  have 
been  the  philosopher's  chief  agents  of  combustion.  On 
a  small  scale  we  can  readily  produce,  with  the  purely 
invisible  rays  of  the  electric  light,  all  that  Archimedes 
is  said  to  have  performed  with  the  sun's  total  radia- 
tion. Placing  behind  the  electric  light  a  small  con- 
cave mirror,  the  rays  are  converged,  the  cone  of  reflected 
rays  and  their  point  of  convergence  being  rendered 
clearly  visible  by  the  dust  always  floating  in  the  air. 
Placing  between  the  luminous  focus  and  the  source  of 
rays  our  solution  of  iodine,  the  light  of  the  cone  is  en- 
tirely cut  away;  but  the  intolerable  heat  experienced 
when  the  hand  is  placed,  even  for  a  moment,  at  the 
dark  focus,  shows  that  the  calorific  rays  pass  unimpeded 
through  the  opaque  solution. 

Almost  anything  that  ordinary  fire  can  effect  may 
be  accomplished  at  the  focus  of  invisible  rays;  the  air 


RADIATION.  47 

at  the  focus  remaining  at  the  same  time  perfectly  cold, 
on  account  of  its  transparency  to  the  heat-rays.  An  air 
thermometer,  with  a  hollow  rock-salt  bulb,  would  be 
unaffected  by  the  heat  of  the  focus:  there  would  be  no 
expansion,  and  in  the  open  air  there  is  no  convection. 
The  ether  at  the  focus,  and  not  the  air,  is  the  sub- 
stance in  which  the  heat  is  embodied.  A  block  of  wood, 
placed  at  the  focus,  absorbs  the  heat,  and  dense  vol- 
umes of  smoke  rise  swiftly  upwards,  showing  the  man- 
ner in  which  the  air  itself  would  rise,  if  the  invisible  rays 
were  competent  to  heat  it.  At  the  perfectly  dark  focus 
dry  paper  is  instantly  inflamed:  chips  of  wood  are 
speedily  burnt  up:  lead,  tin,  and  zinc  are  fused:  and 
disks  of  charred  paper  are  raised  to  vivid  incandescence. 
It  might  be  supposed  that  the  obscure  rays  would  show 
no  preference  for  black  over  white;  but  they  do  show 
a  preference,  and  to  obtain  rapid  combustion,  the  body, 
if  not  already  black,  ought  to  be  blackened.  When 
metals  are  to  be  burned,  it  is  necessary  to  blacken  or 
otherwise  tarnish  them,  so  as  to  diminish  their  reflective 
power.  Blackened  zinc  foil,  when  brought  into  the 
focus  of  invisible  rays,  is  instantly  caused  to  blaze,  and 
burns  with  its  peculiar  purple  light.  Magnesium  wire 
flattened,  or  tarnished  magnesium  ribbon,  also  bursts 
into  flame.  Pieces  of  charcoal  suspended  in  a  receiver 
full  of  oxygen  are  also  set  on  fire  when  the  invisible 
focus  falls  upon  them;  the  dark  rays  after  having 
passed  through  the  receiver,  still  possessing  sufficient 
power  to  ignite  the  charcoal,  and  thus  initiate  the  at- 
tack of  the  oxygen.  If,  instead  of  being  plunged  in 
oxygen,  the  charcoal  be  suspended  in  vacuo,  it  immedi- 
ately glows  at  the  place  where  the  focus  falls. 


48  FRAGMENTS    OF    SCIENCE. 


8.  Transmutation  of  Rays:*  Calorescence. 

Eminent  experimenters  were  long  occupied  in  de- 
monstrating the  substantial  identity  of  light  and  radi- 
ant heat,  and  we  have  now  the  means  of  offering  a  new 
and  striking  proof  of  this  identity.  A  concave  mirror 
produces,  beyond  the  object  which  it  reflects,  an  in- 
verted and  magnified  image  of  the  object.  Withdraw- 
ing, for  example,  our  iodine  solution,  an  intensely 
luminous  inverted  image  of  the  carbon  points  of  the 
electric  light  is  formed  at  the  focus  of  the  mirror  em- 
ployed in  the  foregoing  experiments.  When  the  solu- 
tion is  interposed,  and  the  light  is  cut  away,  what  be- 
comes of  this  image?  It  disappears  from  sight;  but 
an  invisible  thermograph  remains,  and  it  is  only  the 
peculiar  constitution  of  our  eyes  that  disqualifies  us 
from  seeing  the  picture  formed  by  the  calorific  rays. 
Falling  on  white  paper,  the  image  chars  itself  out:  fall- 
ing on  black  paper,  two  holes  are  pierced  in  it,  corre- 
sponding to  the  images  of  the  two  coke  points:  but 
falling  on  a  thin  plate  of  carbon  in  vacuo,  or  upon  a 
thin  sheet  of  platinised  platinum,  either  in  vacuo  or  in 
air,  radiant  heat  is  converted  into  light,  and  the  image 
stamps  itself  in  vivid  incandescence  upon  both  the  car- 
bon and  the  metal.  Eesults  similar  to  those  obtained 
with  the  electric  light  have  also  been  obtained  with 
the  invisible  rays  of  the  lime-light  and  of  the  sun. 

Before  a  Cambridge  audience  it  is  hardly  necessary 
to  refer  to  the  excellent  researches  of  Professor  Stokes 
at  the  opposite  end  of  the  spectrum.  The  above  re- 
sults constitute  a  kind  of  complement  to  his  discoveries. 
Professor  Stokes  named  the  phenomena  which  he  has 

*  I  borrow  this  term  from  Professor  Challis,  '  Philosophical 
Magazine,'  vol.  xii.  p.  521. 


RADIATION.  49 

discovered  and  investigated  Fluorescence;  for  the  new 
phenomena  here  described  I  have  proposed  the  term 
Caloreseence.  He,  by  the  interposition  of  a  proper  me- 
dium, so  lowered  the  refrangibility  of  the  ultra-violet 
rays  of  the  spectrum  as  to  render  them  visible.  Here, 
by  the  interposition  of  the  platinum  foil,  the  refrangi- 
bility of  the  ultra-red  rays  is  so  exalted  as  to  render 
them  visible.  Looking  through  a  prism  at  the  incan- 
descent image  of  the  carbon  points,  the  light  of  the 
image  is  decomposed,  and  a  complete  spectrum  is  ob- 
tained. The  invisible  rays  of  the  electric  light,  re- 
moulded by  the  atoms  of  the  platinum,  shine  thus  visi- 
bly forth;  ultra-red  rays  being  converted  into  red, 
orange,  yellow,  green,  blue,  indigo,  violet,  and  ultra- 
violet ones.  Could  we,  moreover,  raise  the  original 
source  of  rays  to  a  sufficiently  high  temperature,  we 
might  not  only  obtain  from  the  dark  rays  of  such  a 
source  a  single  incandescent  image,  but  from  the  dark 
rays  of  this  image  we  might  obtain  a  second  one,  from 
the  dark  rays  of  the  second  a  third,  and  so  on — a  series 
of  complete  images  and  spectra  being  thus  extracted 
from  the  invisible  emission  of  the  primitive  source.* 

*  On  investigating  the  caloreseence  produced  by  rays  trans- 
mitted through  glasses  of  various  colours,  it  was  found  that  in 
the  case  of  certain  specimens  of  blue  glass,  the  platinum  foil 
glowed  with  ft  pink  or  purplish  light.  The  effect  was  not  sub- 
jective, and  considerations  of  obvious  interest  are  suggested  by 
it.  Different  kinds  of  black  glass  differ  notably  as  to  their  power 
of  transmitting  radiant  heat.  When  thin,  some  descriptions  tint 
the  sun  with  a  greenish  hue ;  others  make  it  appear  a  glowing 
red  without  any  trace  of  green.  The  latter  are  far  more  dia- 
thermic than  the  former.  In  fact.,  carbon  when  perfectly  dis- 
solved and  incorporated  with  a  good  white  glass,  is  highly  trans- 
parent to  the  calorific  rays,  and  by  employing  it  as  an  absorbent 
the  phenomena  of  'caloreseence'  may  be  obtained,  though  in  a 
less  striking  form  than  with  the  iodine.  The  black  glass  chosen 
for  thermometers,  and  intended  to  absorb  completely  the  solar 
heat,  may  entirely  fail  in  this  object,  if  the  glass  in  which  the 


50  FRAGMENTS    OF    SCIENCE. 

9.  Deadness  of  the  Optic  Nerve  to  the  Calorific  Rays. 

The  layer  of  iodine  used  in  the  foregoing  experi- 
ments intercepted  the  rays  of  the  noonday  sun,  No 
trace  of  light  from  the  electric  lamp  was  visible  in  the 
darkest  room,  even  when  a  white  screen  was  placed  at 
the  focus  of  the  mirror  employed  to  concentrate  the 
light.  It  was  thought,  however,  that  if  the  retina  itself 
were  brought  into  the  focus  the  sensation  of  light  might 
be  experienced.  The  danger  of  this  experiment  was 
twofold.  If  the  dark  rays  were  absorbed  in  a  high  de- 
gree by  the  humours  of  the  eye  the  albumen  of  the  hu- 
mours might  coagulate  along  the  line  of  the  rays.  If, 
on  the  contrary,  no  such  high  absorption  took  place,  the 
rays  might  reach  the  retina  with  a  force  sufficient  to  de- 
stroy it.  To  test  the  likelihood  of  these  results,  experi- 
ments were  made  on  water  and  on  a  solution  of  alum, 
and  they  showed  it  to  be  very  improbable  that  in  the 
brief  time  requisite  for  an  experiment  any  serious  dam- 
age could  be  done.  The  eye  was  therefore  caused  to 
approach  the  dark  focus,  no  defence,  in  the  first  in- 
stance, being  provided;  but  the  heat,  acting  upon  the 
parts  surrounding  the  pupil,  could  not  be  borne.  An 
aperture  was  therefore  pierced  in  a  plate  of  metal,  and 
the  eye,  placed  behind  the  aperture,  was  caused  to  ap- 
proach the  point  of  convergence  of  invisible  rays.  The 
focus  was  attained,  first  by  the  pupil  and  afterwards 
by  the  retina.  Eemoving  the  eye,  but  permitting  the 
plate  of  metal  to  remain,  a  sheet  of  platinum  foil  was 

carbon  is  incorporated  be  colourless.  To  render  the  bulb  of  a 
thermometer  a  perfect  absorbent,  the  glass  ought  in  the  first  in- 
stance to  be  green.  Soon  after  the  discovery  of  fluorescence  the 
late  Dr.  William  Allen  Miller  pointed  to  the  lime-light  as  un 
illustration  of  exalted  refrangibility.  Direct  experiments  have 
since  entirely  confirmed  the  view  expressed  at  page  210  of  his 
work  on  '  Chemistry,'  published  in  1855. 


RADIATION.  51 

placed  in  the  position  occupied  by  the  retina  a  moment 
before.  The  platinum  became  red-hot.  No  sensible 
damage  was  done  to  the  eye  by  this  experiment;  no 
impression  of  light  was  produced;  the  optic  nerve  was 
not  even  conscious  of  heat. 

But  the  humours  of  the  eye  are  known  to  be  highly 
impervious  to  the  invisible  calorific  rays,  and  the  ques- 
tion therefore  arises,  *  Did  the  radiation  in  the  forego- 
ing experiment  reach  the  retina  at  all?'  The  answer 
is,  that  the  rays  were  in  part  transmitted  to  the  retina, 
and  in  part  absorbed  by  the  humours.  Experiments 
on  the  eye  of  an  ox  showed  that  the  proportion  of  ob- 
scure rays  which  reached  the  retina  amounted  to  18  per 
cent,  of  the  total  radiation;  while  the  luminous  emis- 
sion from  the  electric  light  amounts  to  no  more  than 
10  per  cent,  of  the  same  total.  Were  the  purely  lumin- 
ous rays  of  the  electric  lamp  converged  by  our  mirror 
to  a  focus,  there  can  be  no  doubt  as  to  the  fate  of  a 
retina  placed  there.  Its  ruin  would  be  inevitable;  and 
yet  this  would  be  accomplished  by  an  amount  of  wave- 
motion  but  little  more  than  half  of  that  which  the 
retina,  without  exciting  consciousness,  bears  at  the  focus 
of  invisible  rays. 

This  subject  will  repay  a  moment's  further  atten- 
tion. At  a  common  distance  of  a  foot  the  visible  radia- 
tion of  the  electric  light  employed  in  these  experiments 
is  800  times  the  light  of  a  candle.  At  the  same  dis- 
tance, the  portion  of  the  radiation  of  the  electric  light 
which  reaches  the  retina,  but  fails  to  excite  vision,  is 
about  1,500  times  the  luminous  radiation  of  the  candle.* 
But  a  candle  on  a  clear  night  can  readily  be  seen  at  a 
distance  of  a  mile,  its  light  at  this  distance  being  less 

*  It  will  be  borne  in  mind  that  the  heat  which  any  ray,  lum- 
inous or  non-luminous,  is  competent  to  generate  is  tho  true  meas- 
ure of  the  energy  of  the  ray. 


52  FKAGMENTS    OF    SCIENCE. 

than  20)OQ10)0oo  of  its  light  at  the  distance  of  a  foot. 
Hence,  to  make  the  candle-light  a  mile  off  equal  in 
power  to  the  non-luminous  radiation  received  from  the 
electric  light  at  a  foot  distance,  its  intensity  would 
have  to  be  multiplied  by  1,500  X  20,000,000,  or  by 
thirty  thousand  millions.  Thus  the  thirty  thousand 
millionth  part  of  the  invisible  radiation  from  the  elec- 
tric light,  received  by  the  retina  at  the  distance  of  a 
foot,  would,  if  slightly  changed  in  character,  be  amply 
sufficient  to  provoke  vision.  Nothing  could  more  forci- 
bly illustrate  that  special  relationship  supposed  by 
Melloni  and  others  to  subsist  between  the  optic  nerve 
and  the  oscillating  periods  of  luminous  bodies.  The 
optic  nerve  responds,  as  it  were,  to  the  waves  with  which 
it  is  in  consonance,  while  it  refuses  to  be  excited  by 
others  of  almost  infinitely  greater  energy,  whose  periods 
of  recurrence  are  not  in  unison  with  its  own. 


10.  Persistence  of  Rays. 

At  an  early  part  of  this  lecture  it  was  affirmed,  that 
when  a  platinum  wire  was  gradually  raised  to  a  state  of 
high  incandescence,  new  rays  were  constantly  added, 
while  the  intensity  of  the  old  ones  was  increased.  Thus, 
in  Dr.  Draper's  experiments,  the  rise  of  temperature 
that  generated  the  orange,  yellow,  green,  and  blue  aug- 
mented the  intensity  of  the  red.  What  is  true  of  the 
red  is  true  of  every  other  ray  of  the  spectrum,  visible 
and  invisible.  We  cannot  indeed  see  the  augmentation 
of  intensity  in  the  region  beyond  the  red,  but  we  can 
measure  it  and  express  it  numerically.  With  this  view 
the  following  experiment  was  performed:  A  spiral  of 
platinum  wire  was  surrounded  by  a  small  glass  globe 
to  protect  it  from  currents  of  air;  through  an  orifice 
in  the  globe  the  rays  could  pass  from  the  spiral  and 


RADIATION.  53 

fall  afterwards  upon  a  thermo-electric  pile.  Placing 
in  front  of  the  orifice  an  opaque  solution  of  iodine, 
the  platinum  was  gradually  raised  from  a  low  dark 
heat  to  the  fullest  incandescence,  with  the  following 
results: — 

Appearance  Energy  of 
of  spiral                                                                  obscure  radiation 

Dark 1 

Dark,  but  hotter 3 

Dark,  but  still  hotter 5 

Dark,  but  still  hotter 10 

.Feeble  red 19 

Dull  red 25 

Red 37 

Full  red 62 

Orange 89 

Bright  orange 144 

Yellow 202 

White 276 

Intense  white 440 

Thus  the  augmentation  of  the  electric  current, 
which  raises  the  wire  from  its  primitive  dark  condition 
to  an  intense  white  heat,  exalts  at  the  same  time  the 
energy  of  the  obscure  radiation,  until  at  the  end  it  is 
fully  440  times  what  it  was  at  the  beginning. 

What  has  been  here  proved  true  of  the  totality  of 
the  ultra-red  rays  is  true  for  each  of  them  singly.  Plac- 
ing our  linear  thermo-electric  pile  in  any  part  of  the 
ultra-red  spectrum,  it  may  be  proved  that  a  ray  once 
emitted  continues  to  be  emitted  with  increased  energy 
as  the  temperature  is  augmented.  The  platinum  spiral, 
so  often  referred  to,  being  raised  to  whiteness  by  an 
electric  current,  a  brilliant  spectrum  was  formed  from 
its  light.  A  linear  thermo-electric  pile  was  placed  in 
the  region  of  obscure  rays  beyond  the  red,  and  by  di- 
minishing the  current  the  spiral  was  reduced  to  a  low 
temperature.  It  was  then  caused  to  pass  through  vari- 


54  FRAGMENTS    OF    SCIENCE. 

ous  degrees  of  darkness  and  incandescence,  with  the 
following  results: — 

Appearance  .  Energy  of 

of  spiral  obscure  rays 

Dark        .        .        .        .        ....        .          1 

Dark 6 

Faint  red .        .        .    •   .        .        .        .        .        10 

Dull  red   .        .        .       ....        .        13 

Red  .        .        .        .  *      .  .        .        .        18 

Full  red    .        . 27 

Orange 60 

Yellow .        .        93 

White      ...       ...'.'.       .122 

Here,  as  in  the  former  case,  the  dark  and  bright 
radiations  reached  their  maximum  together;  as  the  one 
augmented,  the  other  augmented,  until  at  last  the  en- 
ergy of  the  obscure  rays  of  the  particular  refrangibility 
here  chosen,  became  122  times  what  it  was  at  first.  To 
reach  a  white  heat  the  wire  has  to  pass  through  all  the 
stages  of  invisible  radiation,  but  in  its  most  brilliant 
condition  it  embraces,  in  an  intensified  form,  the  rays 
of  all  those  stages. 

And  thus  it  is  with  all  other  kinds  of  matter,  as  far 
as  they  have  hitherto  been  examined.  Coke,  whether 
brought  to  a  white  heat  by  the  electric  current,  or  by 
the  oxyhydrogen  jet,  pours  out  invisible  rays  with  aug- 
mented energy,  as  its  light  is  increased.  The  same  is 
true  of  lime,  bricks,  and  other  substances.  It  is  true 
of  all  metals  which  are  capable  of  being  heated  to  in- 
candescence. It  also  holds  good  for  phosphorus  burn- 
ing in  oxygen.  Every  gush  of  dazzling  light  has  asso- 
ciated with  it  a  gush  of  invisible  radiant  heat,  which 
far  transcends  the  light  in  energy.  This  condition  of 
things  applies  to  all  bodies  capable  of  being  raised  to  a 
white  heat,  either  in  the  solid  or  the  molten  condition. 
It  would  doubtless  also  apply  to  the  luminous  fogs 


RADIATION.  55 

formed  by  the  condensation  of  incandescent  vapours. 
In  such  cases  when  the  curve  representing  the  radiant 
energy  of  the  body  is  constructed,  the  obscure  radia- 
tion towers  upwards  like  a  mountain,  the  luminous  radi- 
ation resembling  a  mere  '  spur '  at  its  base.  From  the 
very  brightness  of  the  light  of  some  of  the  fixed  stars 
we  may  infer  the  intensity  of  that  dark  radiation,  which 
is  the  precursor  and  inseparable  associate  of  their  lu- 
minous rays. 

We  thus  find  the  luminous  radiation  appearing 
when  the  radiant  body  has  attained  a  certain  temper- 
ature; or,  in  other  words,  when  the  vibrating  atoms  of 
the  body  have  attained  a  certain  width  of  swing.  In 
solid  and  molten  bodies  a  certain  amplitude  cannot  be 
surpassed  without  the  introduction  of  periods  of  vibra- 
tion, which  provoke  the  sense  of  vision.  How  are  we 
to  figure  this?  If  permitted  to  speculate,  we  might 
ask,  are  not  these  more  rapid  vibrations  the  progeny  of 
the  slower?  Is  it  not  really  the  mutual  action  of  the 
atoms,  when  they  swing  through  very  wide  spaces,  and 
thus  encroach  upon  each  other,  that  causes  them  to 
tremble  in  quicker  periods?  If  so,  whatever  be  the 
agency  by  which  the  large  swinging  space  is  obtained, 
we  shall  have  light-giving  vibrations  associated  with  it. 
It  matters  not  whether  the  large  amplitudes  be  pro- 
duced by  the  strokes  of  a  hammer,  or  by  the  blows  of 
the  molecules  of  a  non-luminous  gas,  like  air  at  some 
height  above  a  gas-flame;  or  by  the  shock  of  the  ether 
particles  when  transmitting  radiant  heat.  The  result 
in  all  cases  will  be  incandescence.  Thus,  the  invisible 
waves  of  our  filtered  electric  beam  may  be  regarded  as 
generating  synchronous  vibrations  among  the  atoms  of 
the  platinum  on  which  they  impinge;  but,  once  these 
vibrations  have  attained  a  certain  amplitude,  the  mu- 
tual jostling  of  the  atoms  produces  quicker  tremors,  and 


56  FRAGMENTS    OF    SCIENCE. 

the  light-giving  waves  follow  as  the  necessary  product 
of  the  heat-giving  ones. 


11.  Absorption  of  Radiant  Heat  ly  Vapours  and 
Odours. 

"We  commenced  the  demonstrations  brought  for- 
ward in  this  lecture  by  experiments  on  permanent  gases, 
and  we  have  now  to  turn  our  attention  to  the  vapours  of 
volatile  liquids.  Here,  as  in  the  case  of  the  gases,  vast 
differences  have  been  proved  to  exist  between  various 
kinds  of  molecules,  as  regards  their  power  of  intercept- 
ing the  calorific  waves.  While  some  vapours  allow  the 
waves  a  comparatively  free  passage,  the  faintest  mixture 
of  other  vapours  causes  a  deflection  of  the  magnetic 
needle.  Assuming  the  absorption  effected  by  air,  at  a 
pressure  of  one  atmosphere,  to  be  unity,  the  following 
are  the  absorptions  effected  by  a  series  of  vapours  at  a 
pressure  of  -g^th  of  an  atmosphere:  — 


Name  of  vapour  Absorption 

Bisulphide  of  carbon        .....  47 

Iodide  of  methyl      ......  115 

Benzol      ........  136 

Amylene  ........  321 

Sulphuric  ether        ......  440 

Formic  ether   ........  548 

Acetic  ether     .......  612 

Bisulphide  of  carbon  is  the  most  transparent  vapour 
in  this  list;  and  acetic  ether  the  most  opaque;  ^th  of 
an  atmosphere  of  the  former,  however,  produces  47 
times  the  effect  of  a  whole  atmosphere  of  air,  while 
•5*5  th  of  an  atmosphere  of  the  latter  produces  612  times 
the  effect  of  a  whole  atmosphere  of  air.  Reducing  dry 
air  to  the  pressure  of  the  acetic  ether  here  employed, 
And  comparing  them  then  together,  the  quantity  of 


RADIATION. 


57 


wave-motion  intercepted  by  the  ether  would  be  many 
thousand  times  that  intercepted  by  the  air. 

Any  one  of  these  vapours  discharged  into  the  free 
atmosphere,  in  front  of  a  body  emitting  obscure  rays, 
intercepts  more  or  less  of  the  radiation.  A  similar  ef- 
fect is  produced  by  perfumes  diffused  in  the  air,  though 
their  attenuation  is  known  to  be  almost  infinite.  Car- 
rying, for  example,  a  current  of  dry  air  over  bibulous 
paper,  moistened  by  patchouli,  the  scent  tak«n  up  by 
the  current  absorbs  30  times  the  quantity  of  heat  inter- 
cepted by  the  air  which  carries  it;  and  yet  patchouli 
acts  more  feebly  on  radiant  heat  than  any  other  per- 
fume yet  examined.  Here  follow  the  results  obtained 
with  various  essential  oils,  the  odour,  in  each  case,  being 
carried  by  a  current  of  dry  air  into  the  tube  already  em- 
ployed for  gases  and  vapours: — 


Name  of  perfume 
Patchouli 
Sandal  wood    . 
Geranium 
Oil  of  cloves 
Otto  of  roses     . 


Absorption 


Bergamot 44 

Neroli 47 

Lavender .        .        60 

Lemon      ...  .65 

Portugal  .  67 

Thyme     ...  68 

Rosemary         .        .  74 

Oil  of  laurel     .  80 

Camomile  flowers    .  87 

Cassia       ...  109 

Spikenard         ....  .        »  ..856 

Aniseed    ........      872 

Thus  the  absorption  by  a  tube  full  of  dry  air  being 
1,  that  of  the  odour  of  patchouli  diffused  in  it  is  30, 
that  of  lavender  60,  that  of  rosemary  74,  whilst  that  of 
aniseed  amounts  to  372.  It  would  be  idle  to  speculate 
on  the  quantities  of  matter  concerned  in  these  actions. 


58  FRAGMENTS    OF    SCIENCE. 


12.  Aqueous  Vapour  in  relation  to  the  Terrestrial 
Temperatures. 

"VVe  are  now  fully  prepared  for  a  result  which,  with- 
out such  preparation,  might  appear  incredible.  Water 
is,  to  some  extent,  a  volatile  body,  and  our  atmosphere, 
resting  as  it  does  upon  the  surface  of  the  ocean,  receives 
from  it  a  continual  supply  of  aqueous  vapour.  It 
would  be  an  error  to  confound  clouds  or  fog  or  any 
visible  mist  with  the  vapour  of  water,  which  is  a  per- 
fectly impalpable  gas,  diffused,  even  on  the  clearest 
days,  throughout  the  atmosphere.  Compared  with  the 
great  body  of  the  air,  the  aqueous  vapour  it  contains 
is  of  almost  infinitesimal  amount,  99^  out  of  every  100 
parts  of  the  atmosphere  being  composed  of  oxygen  and 
nitrogen.  In  the  absence  of  experiment,  we  should 
never  think  of  ascribing  to  this  scant  and  varying  con- 
stituent any  important  influence  on  terrestrial  radia- 
tion; and  yet  its  influence  is  far  more  potent  than  that 
of  the  great  body  of  the  air.  To  say  that  on  a  day  of 
average  humidity  in  England,  the  atmospheric  vapour 
exerts  100  times  the  action  of  the  air  itself,  would 
certainly  be  an  understatement  of  the  fact.  Compar- 
ing a  single  molecule  of  aqueous  vapour  with  an 
atom  of  either  of  the  main  constituents  of  our  atmos- 
phere, I  am  not  prepared  to  say  how  many  thousand 
times  the  action  of  the  former  exceeds  that  of  the 
latter. 

But  it  must  be  borne  in  mind  that  these  large  num- 
bers depend,  in  part,  on  the  extreme  feebleness  of  the 
air;  the  power  of  aqueous  vapour  seems  vast,  because 
that  of  the  air  with  which  it  is  compared  is  infinites- 
imal. Absolutely  considered,  however,  this  substance, 
notwithstanding  its  small  specific  gravity,  exercises  a 


RADIATION.  59 

very  potent  action.  Probably  from  10  to  15  per  cent, 
of  the  heat  radiated  from  the  earth  is  absorbed  within 
10  or  20  feet  of  the  earth's  surface.  This  must  evidently 
be  of  the  utmost  consequence  to  the  life  of  the  world. 
Imagine  the  superficial  molecules  of  the  earth  agitated 
with  the  motion  of  heat,  and  imparting  it  to  the  sur- 
rounding ether;  this  motion  would  be  carried  rapidly 
away,  and  lost  for  ever  to  our  planet,  if  the  waves  of 
ether  had  nothing  but  the  air  to  contend  with  in  their 
outward  course.  But  the  aqueous  vapour  takes  up  the 
motion,  and  becomes  thereby  heated,  thus  wrapping 
the  earth  like  a  warm  garment,  and  protecting  its 
surface  from  the  deadly  chill  which  it  would  other- 
wise sustain.  Various  philosophers  have  speculated 
on  the  influence  of  an  atmospheric  envelope.  De 
Saussure,  Fourier,  M.  Pouillet,  and  Mr.  Hopkins  have, 
one  and  all,  enriched  scientific  literature  with  contribu- 
tions on  this  subject,  but  the  considerations  which  these 
eminent  men  have  applied  to  atmospheric  air,  have,  if 
my  experiments  be  correct,  to  be  transferred  to  the 
aqueous  vapour. 

The  observations  of  meteorologists  furnish  impor- 
tant, though  hitherto  unconscious  evidence  of  the  in- 
fluence of  this  agent.  Wherever  the  air  is  dry  we  are 
liable  to  daily  extremes  of  temperature.  By  day,  in 
such  places,  the  sun's  heat  reaches  the  earth  unimpeded, 
and  renders  the  maximum  high;  by  night,  on  the  other 
hand,  the  earth's  heat  escapes  unhindered  into  space, 
and  renders  the  minimum  low.  Hence  the  difference 
between  the  maximum  and  minimum  is  greatest 
where  the  air  is  driest.  In  the  plains  of  India,  on  the 
heights  of  the  Himalaya,  in  central  Asia,  in  Australia — 
wherever  drought  reigns,  we  have  the  heat  of  day  forci- 
bly contrasted  with  the  chill  of  night.  In  the  Sahara 
itself,  when  the  sun's  rays  cease  to  impinge  on  the 


60  FRAGMENTS    OF    SCIENCE. 

burning  soil,  the  temperature  runs  rapidly  down  to 
freezing,  because  there  is  no  vapour  overhead  to  check 
the  calorific  drain.  And  here  another  instance  might 
be  added  to  the  numbers  already  known,  in  which 
nature  tends  as  it  were  to  check  her  own  excess.  By 
nocturnal  refrigeration,  the  aqueous  vapour  of  the  air 
i&  condensed  to  water  on  the  surface  of  the  earth;  and, 
as  only  the  superficial  portions  radiate,  the  act  of  con- 
densation makes  water  the  radiating  body.  Now  ex- 
periment proves  that  to  the  rays  emitted  by  water, 
aqueous  vapour  is  especially  opaque.  Hence  the  very 
act  of  condensation,  consequent  on  terrestrial  cooling, 
becomes  a  safeguard  to  the  earth,  imparting  to  its  radia- 
tion that  particular  character  which  renders  it  most 
liable  to  be  prevented  from  escaping  into  space. 

It  might  however  be  urged  that,  inasmuch  as  we 
derive  all  our  heat  from  the  sun,  the  selfsame  covering 
which  protects  the  earth  from  chill  must  also  shut  out 
the  solar  radiation.  This  is  partially  true,  but  only 
partially;  the  sun's  rays  are  different  in  quality  from 
the  earth's  rays,  and  it  does  not  at  all  follow  that  the 
substance  which  absorbs  the  one  must  necessarily  absorb 
the  other.  Through  a  layer  of  water,  for  example,  one 
tenth  of  an  inch  in  thickness,  the  sun's  rays  are  trans- 
mitted with  comparative  freedom;  but  through  a  layer 
half  this  thickness,  as  Melloni  has  proved,  no  single  ray 
from  the  warmed  earth  could  pass.  In  like  manner, 
the  sun's  rays  pass  with  comparative  freedom  through 
the  aqueous  vapour  of  the  air:  the  absorbing  power  of 
this  substance  being  mainly  exerted  upon  the  invisible 
heat  that  endeavours  to  escape  from  the  earth.  In 
consequence  of  this  differential  action  upon  solar  and 
terrestrial  heat,  the  mean  temperature  of  our  planet  is 
higher  than  is  due  to  its  distance  from  the  sun. 


RADIATION.  61 


13.  Liquids  and  their  Vapours  in  relation  to 
Radiant  Heat. 

The  deportment  here  assigned  to  atmospheric  va- 
pour has  been  established  by  direct  experiments  on  air 
taken  from  the  streets  and  parks  of  London,  from  the 
downs  of  Epsom,  from  the  hills  and  sea-beach  of  the 
Isle  of  Wight,  and  also  by  experiments  on  air  in  the 
first  instance  dried,  and  afterwards  rendered  artificially 
humid  by  pure  distilled  water.  It  has  also  been  es- 
tablished in  the  following  way:  Ten  volatile  liquids 
were  taken  at  random  and  the  power  of  these  liquids,  at 
a  common  thickness,  to  intercept  the  waves  of  heat, 
was  carefully  determined.  The  vapours  of  the  liquids 
were  next  taken,  in  quantities  proportional  to  the  quan- 
tities of  liquid,  and  the  power  of  the  vapours  to  inter- 
cept the  waves  of  heat  was  also  determined.  Com- 
mencing with  the  substance  which  exerted  the  least 
absorptive  power,  and  proceeding  onwards  to  the 
most  energetic,  the  following  order  of  absorption  was 
observed: — 

Liquids  Vapours 

Bisulphide  of  carbon.  Bisulphide  of  carbon. 

Chloroform.  Chloroform. 

Iodide  of  methyl.  Iodide  of  methyl 

Iodide  of  ethyl.  Iodide  of  ethyl 

Benzol.  Benzol. 

Amylene.  Amylene. 

Sulphuric  ether.  Sulphuric  ether. 

Acetic  ether.  Acetic  ether. 

Formic  ether.  Formic  ether. 

Alcohol.  Alcohol. 
Water. 

We  here  find  the  order  of  absorption  in  both  cases 
to  be  the  same.  We  have  liberated  the  molecules  from 
the  bonds  which  trammel  them  more  or  less  in  a  liquid 


62  FRAGMENTS    OF    SCIENCE. 

condition;  but  this  change  in  their  state  of  aggregation 
does  not  change  their  relative  powers  of  absorption. 
Nothing  could  more  clearly  prove  that  the  act  of  ab- 
sorption depends  upon  the  individual  molecule,  which 
equally  asserts  its  power  in  the  liquid  and  the  gaseous 
state.  We  may  safely  conclude  from  the  above  table 
that  the  position  of  a  vapour  is  determined  by  that  of 
its  liquid.  Now  at  the  very  foot  of  the  list  of  liquids 
stands  water,  signalising  itself  above  all  others  by  its 
enormous  power  of  absorption.  And  from  this  fact, 
even  if  no  direct  experiment  on  the  vapour  of  water 
had  ever  been  made,  we  should  be  entitled  to  rank  that 
vapour  as  our  most  powerful  absorber  of  radiant  heat. 
Its  attenuation,  however,  diminishes  its  action.  I  have 
proved  that  a  shell  of  air  two  inches  in  thickness  sur- 
rounding our  planet,  and  saturated  with  the  vapour  of 
sulphuric  ether,  would  intercept  35  per  cent,  of  the 
earth's  radiation.  And  though  the  quantity  of  aque- 
ous vapour  necessary  to  saturate  air  is  much  less  than 
the  amount  of  sulphuric  ether  vapour  which  it  can  sus- 
tain, it  is  still  extremely  probable  that  the  estimate 
already  made  of  the  action  of  atmospheric  vapour  with- 
in 10  feet  of  the  earth's  surface,  is  under  the  mark;  and 
that  we  are  indebted  to  this  wonderful  substance,  to  an 
extent  not  accurately  determined,  but  certainly  far  be- 
yond what  has  hitherto  been  imagined,  for  the  tem- 
perature now  existing  at  the  surface  of  the  globe. 


14.  Reciprocity  of  Radiation  and  Absorption. 

Throughout  the  reflections  which  have  hitherto  oc- 
cupied us,  the  image  before  the  mind  has  been  that  of 
a  radiant  source  sending  forth  calorific  waves,  which  on 
passing  among  the  molecules  of  a  gas  or  vapour  were 


RADIATION.  G3 

intercepted  by  those  molecules  in  various  degrees.  lu 
all  cases  it  was  the  transference  of  motion  from  the 
ether  to  the  comparatively  quiescent  molecules  of  the 
gas  or  vapour  that  occupied  our  thoughts.  We  have 
now  to  change  the  form  of  our  conception,  and  to  figure 
these  molecules  not  as  absorbers  but  as  radiators,  not  as 
the  recipients  but  as  the  originators  of  wave-motion. 
That  is  to  say,  we  must  figure  them  vibrating,  and  gen- 
erating in  the  surrounding  ether  undulations  which 
speed  through  it  with  the  velocity  of  light.  Our  object 
now  is  to  enquire  whether  the  act  of  chemical  combina- 
tion, which  proves  so  potent  as  regards  the  phenomena 
of  absorption,  does  not  also  manifest  its  power  in  the 
phenomena  of  radiation.  For  the  examination  of  this 
question  it  is  necessary,  in  the  first  place,  to  heat  our 
gases  and  vapours  to  the  same  temperature,  and  then 
examine  their  power  of  discharging  the  motion  thus 
imparted  to  them  upon  the  ether  in  which  they  swing. 

A  heated  copper  ball  was  placed  above  a  ring  gas- 
burner  possessing  a  great  number  of  small  apertures, 
the  burner  being  connected  by  a  tube  with  vessels  con- 
taining the  various  gases  to  be  examined.  By  gentle 
pressure  the  gases  were  forced  through  the  orifices  of 
the  burner  against  the  copper  ball,  where  each  of  them, 
being  heated,  rose  in  an  ascending  column.  A  thermo- 
electric pile,  entirely  screened  from  the  hot  ball,  was 
exposed  to  the  radiation  of  the  warm  gas,  while  the 
deflection  of  a  magnetic  needle  connected  with  the  pile 
declared  the  energy  of  the  radiation. 

By  this  mode  of  experiment  it  was  proved  that  the 
selfsame  molecular  arrangement  which  renders  a  gas  a 
powerful  absorber,  renders  it  a  powerful  radiator — that* 
the  atom  or  molecule  which  is  competent  to  intercept 
the  calorific  waves  is,  in  the  same  degree,  competent  to 
send  them  forth.  Thus,  while  the  atoms  of  elementary 


64  FKAGMENTS    OF    SCIENCE. 

gases  proved  themselves  unable  to  emit  any  sensible 
amount  of  radiant  heat,  the  molecules  of  compound 
gases  were  shown  to  be  capable  of  powerfully  disturbing 
the  surrounding  ether.  By  special  modes  of  experiment 
the  same  was  proved  to  hold  good  for  the  vapours  of 
volatile  liquids,  the  radiative  power  of  every  vapour 
being  found  proportional  to  its  absorptive  power. 

The  method  of  experiment  here  pursued,  though 
not  of  the  simplest  character,  is  still  easy  to  grasp. 
When  air  is  permitted  to  rush  into  an  exhausted  tube, 
the  temperature  of  the  air  is  raised  to  a  degree  equi- 
valent to  the  vis  viva  extinguished.*  Such  air  is  said 
to  be  dynamically  heated,  and,  if  pure,  it  shows  itself 
incompetent  to  radiate,  even  when  a  rock-salt  window 
is  provided  for  the  passage  of  its  rays.  But  if  instead 
of  being  empty  the  tube  contain  a  small  quantity  of 
vapour,  the  warmed  air  communicates  its  heat  by  con- 
tact to  the  vapour,  the  molecules  of  which  convert  into 
the  radiant  form  the  heat  imparted  to  them  by  the 
atoms  of  the  air.  By  this  process  also,  which  I  have 
called  Dynamic  Eadiation,  the  reciprocity  of  radiation 
and  absorption  has  been  conclusively  proved.f 

In  the  excellent  researches  of  Leslie,  De  la  Pro- 
vostaye  and  Desains,  and  Balfour  Stewart,  the  same 
reciprocity,  as  regards  solid  bodies,  has  been  variously 
illustrated;  while  the  labours,  theoretical  and  experi- 
mental, of  Kirchhoff  have  given  this  subject  a  won- 
derful expansion,  and  enriched  it  by  applications  of  the 
highest  kind.  To  their  results  are  now  to  be  added 
the  foregoing,  whereby  gases  and  vapours,  which  have 

*  See  page  15  for  a  definition  of  vis  viva. 

f  When  heated  air  imparts  its  motion  to  another  gas  or  va- 
pour, the  transference  of  heat  is  accompanied  by  a  change  of 
vibrating  period.  The  Dynamic  Radiation  of  vapours  is  ren- 
dered possible  by  this  transmutation  of  vibrations. 


RADIATION.  65 

been  hitherto  thought  inaccessible  to  experiments  with 
the  thermo-electric  pile,  are  proved  by  it  to  exhibit 
the  indissoluble  duality  of  radiation  and  absorption, 
the  influence  of  chemical  combination  on  Doth  be- 
ing exhibited  in  the  most  decisive  and  extraordinary 
way. 


15.  Influence  of  Vibrating  Period  and  Molecular  Form. 
Physical  Analysis  of  the  Human  Breath. 

In  the  foregoing  experiments  with  gases  and  va- 
pours we  have  employed  throughout  invisible  rays,  and 
found  some  of  these  bodies  so  impervious  to  radiant 
heat,  that  in  lengths  of  a  few  feet  they  intercept  every 
ray  as  effectually  as  a  layer  of  pitch.  The  substances, 
however,  which  show  themselves  thus  opaque  to  radiant 
heat  are  perfectly  transparent  to  light.  Now  the  rays 
of  light  differ  from  those  of  invisible  heat  merely  in 
point  of  period,  the  former  failing  to  affect  the  retina 
because  their  periods  of  recurrence  are  too  slow.  Hence, 
in  some  way  or  other,  the  transparency  of  our  gases  and 
vapours  depends  upon  the  periods  of  the  waves  which 
impinge  upon  them.  What  is  the  nature  of  this  de- 
pendence? The  admirable  researches  of  Kirchhoff  help 
us  to  an  answer.  The  atoms  and  molecules  of  every  gas 
have  certain  definite  rates  of  oscillation,  and  those  waves 
of  ether  are  most  copiously  absorbed  whose  periods  of 
recurrence  synchronise  with  those  of  the  atomic  groups 
amongst  which  they  pass.  Thus,  when  we  find  the  in- 
visible rays  absorbed  and  the  visible  ones  transmitted 
by  a  layer  of  gas,  we  conclude  that  the  oscillating 
periods  of  the  atoms  constituting  the  gaseous  molecules 
coincide  with  those  of  the  invisible,  and  not  with  those 
of  the  visible  spectrum. 


66  FEAGMENTS    OF    SCIENCE. 

It  requires  some  discipline  of  the  imagination  to 
form  a  clear  picture  of  this  process.  Such  a  picture  is, 
however,  possible,  and  ought  to  be  obtained.  When 
the  waves  of  ether  impinge  upon  molecules  whose 
periods  of  vibration  coincide  with  the  recurrence  of  the 
undulations,  the  timed  strokes  of  the  waves  augment 
the  vibration  of  the  molecules,  as  a  heavy  pendulum  is 
set  in  motion  by  well-timed  puffs  of  breath.  Millions 
of  millions  of  shocks  are  received  every  second  from  the 
calorific  waves;  and  it  is  not  difficult  to  see  that  as 
every  wave  arrives  just  in  time  to  repeat  the  action  of 
its  predecessor,  the  molecules  must  finally  be  caused  to 
swing  through  wider  spaces  than  if  the  arrivals  were 
not  so  timed.  In  fact,  it  is  not  difficult  to  see  that  an 
assemblage  of  molecules,  operated  upon  by  contending 
waves,  might  remain  practically  quiescent.  This  is 
actually  the  case  when  the  waves  of  the  visible  spectrum 
pass  through  a  transparent  gas  or  vapour.  There  is 
here  no  sensible  transference  of  motion  from  the  ether 
to  the  molecules;  in  other  words,  there  is  no  sensible 
absorption  of  heat. 

One  striking  example  of  the  influence  of  period  may 
be  here  recorded.  Carbonic  acid  gas  is  one  of  the 
feeblest  absorbers  of  the  radiant  heat  emitted  by  solid 
bodies.  It  is,  for  example,  to  a  great  extent  transparent 
to  the  rays  emitted  by  the  heated  copper  plate  already 
referred  to.  There  are,  however,  certain  rays,  com- 
paratively few  in  number,  emitted  by  the  copper,  to 
which  the  carbonic  acid  is  impervious;  and  could  we 
obtain  a  source  of  heat  emitting  such  rays  only,  we 
should  find  carbonic  acid  more  opaque  to  the  radiation 
from  that  source,  than  any  other  gas.  Such  a  source  is 
actually  found  in  the  flame  of  carbonic  oxide,  where 
hot  carbonic  acid  constitutes  the  main  radiating  body. 
Of  the  rays  emitted  by  our  heated  plate  of  copper, 


RADIATION.  67 

olefiant  gas  absorbs  ten  times  the  quantity  absorbed  by 
carbonic  acid.  Of  the  rays  emitted  by  a  carbonic  oxide 
flame,  carbonic  acid  absorbs  twice  as  much  as  olefiant 
gas.  This  wonderful  change  in  the  power  of  the  for- 
mer, as  an  absorber,  is  simply  due  to  the  fact,  that  the 
periods  of  the  hot  and  cold  carbonic  acid  are  identical, 
and  that  the  waves  from  the  flame  freely  transfer  their 
motion  to  the  molecules  which  synchronise  with  them. 
Thus  it  is  that  the  tenth  of  an  atmosphere  of  carbonic 
acid,  enclosed  in  a  tube  four  feet  long,  absorbs  60  per 
cent,  of  the  radiation  from  a  carbonic  oxide  flame,  while 
one-thirtieth  of  an  atmosphere  absorbs  48  per  cent,  of 
the  heat  from  the  same  source. 

In  fact,  the  presence  of  the  minutest  quantity  of  car- 
bonic acid  may  be  detected  by  its  action  on  the  rays 
from  the  carbonic  oxide  flame.  Carrying,  for  example, 
the  dried  human  breath  into  a  tube  four  feet  long,  the 
absorption  there  effected  by  the  carbonic  acid  of  the 
breath  amounts  to  50  per  cent,  of  the  entire  radiation. 
Radiant  heat  may  indeed  be  employed  as  a  means  of 
determining  practically  the  amount  of  carbonic  acid 
expired  from  the  lungs.  My  late  assistant,  Mr.  Bar- 
rett, while  under  my  direction,  made  this  determina- 
tion. The  absorption  produced  by  the  breath  freed 
from  its  moisture,  but  retaining  its  carbonic  acid,  was 
first  determined.  Carbonic  acid,  artificially  prepared, 
was  then  mixed  with  dry  air  in  such  proportions  that 
the  action  of  the  mixture  upon  the  rays  of  heat  was  the 
same  as  that  of  the  dried  breath.  The  percentage  of 
the  former  being  known,  immediately  gave  that  of  the 
latter.  The  same  breath,  analysed  chemically  by  Dr. 
Frankland,  and  physically  by  Mr.  Barrett,  gave  the  fol- 
lowing results: — 


68  FKAGMENTS    OF    SCIENCE. 

Percentage  of  Carbonic  Acid  in  the  Human  Breath. 

Chemical  analysis  Physical  analysis 

4-66 4-56 

5-33 5-22 

It  is  thus  proved  that  in  the  quantity  of  ethereal 
motion  which  it  is  competent  to  take  up,  we  have  a  prac- 
tical measure  of  the  carbonic  acid  of  the  breath,  and 
hence  of  the  combustion  going  on  in  the  human  lungs. 

Still  this  question  of  period,  though  of  the  utmost 
importance,  is  not  competent  to  account  for  the  whole 
of  the  observed  facts.  The  ether,  as  far  as  we  know, 
accepts  vibrations  of  all  periods  with  the  same  readiness. 
To  it  the  oscillations  of  an  atom  of  free  oxygen  are  just 
as  acceptable  as  those  of  the  atoms  in  a  molecule  of 
olefiant  gas;  that  the  vibrating  oxygen  then  stands  so 
far  below  the  olefiant  gas  in  radiant  power  must  be  re- 
ferred not  to  period,  but  to  some  other  peculiarity.  The 
atomic  group  which  constitutes  the  molecule  of  olefiant 
gas,  produces  many  thousand  times  the  disturbance 
caused  by  the  oxygen,  it  may  be  because  the  group  is 
able  to  lay  a  vastly  more  powerful  hold  upon  the  ether 
than  the  single  atome  can.  Another,  and  probably 
very  potent  cause  of  the  difference  may  be,  that  the 
vibrations,  being  those  of  the  constituent  atoms  of  the 
molecule,*  are  generated  in  highly  condensed  ether, 
which  acts  like  condensed  air  upon  sound.  But  what- 
ever may  be  the  fate  of  these  attempts  to  visualise  the 
physics  of  the  process,  it  will  still  remain  true,  that  to 
account  for  the  phenomena  of  radiation  and  absorption 
we  must  take  into  consideration  the  shape,  size,  and  con- 
dition of  the  ether  within  the  molecules,  by  which  the 
external  ether  is  disturbed. 

*  See  '  Physical  Considerations,'  Art.  iv.  p.  102. 


RADIATION.  69 


16.  Summary  and  Conclusion. 

Let  us  now  cast  a  momentary  glance  over  the  ground 
that  we  have  left  behind.  The  general  nature  of  light 
and  heat  was  first  briefly  described:  the  compounding 
of  matter  from  elementary  atoms,  and  the  influence  of 
the  act  of  combination  on  radiation  and  absorption, 
were  considered  and  experimentally  illustrated. 
Through  the  transparent  elementary  gases  radiant  heat 
was  found  to  pass  as  through  a  vacuum,  while  many  of 
the  compound  gases  presented  almost  impassable  ob- 
stacles to  the  calorific  waves.  This  deportment  of  the 
simple  gases  directed  our  attention  to  other  elementary 
bodies,  the  examination  of  which  led  to  the  discovery 
that  the  element  iodine,  dissolved  in  bisulphide  of  car- 
bon, possesses  the  power  of  detaching,  with  extraor- 
dinary sharpness,  the  light  of  the  spectrum  from  its 
heat,  intercepting  all  luminous  rays  up  to  the  extreme 
red,  and  permitting  the  calorific  rays  beyond  the  red  to 
pass  freely  through  it.  This  substance  was  then  em- 
ployed to  filter  the  beams  of  the  electric  light,  and'to 
form  foci  of  invisible  rays  so  intense  as  to  produce  al- 
most all  the  effects  obtainable  in  an  ordinary  fire.  Com- 
bustible bodies  were  burnt,  and  refractory  ones  were 
raised  to  a  white  heat,  by  the  concentrated  invisible  rays. 
Thus,  by  exalting  their  ref  rangibility,  the  invisible  rays 
of  the  electric  light  were  rendered  visible,  and  all  the 
colours  of  the  solar  spectrum  were  extracted  from  utter 
darkness.  The  extreme  richness  of  the  electric  light  in 
invisible  rays  of  low  ref  rangibility  was  demonstrated, 
one-eighth  only  of  its  radiation  consisting  of  luminous 
rays.  The  deadness  of  the  optic  nerve  to  those  invisible 
rays  was  proved,  and  experiments  were  then  added  to 
show  that  the  bright  and  the  dark  rays  of  a  solid  body, 


70  FEAGMENTS    OF    SCIENCE. 

raised  gradually  to  incandescence,  are  strengthened  to- 
gether; intense  dark  heat  being  an  invariable  accom- 
paniment of  intense  white  heat.  A  sun  could  not  be 
formed,  or  a  meteorite  rendered  luminous,  on  any  other 
condition.  The  light-giving  rays  constituting  only  a 
small  fraction  of  the  total  radiation,  their  unspeakable 
importance  to  us  is  due  to  the  fact,  that  their  periods 
are  attuned  to  the  special  requirements  of  the  eye. 

Among  the  vapours  of  volatile  liquids  vast  differ- 
ences were  also  found  to  exist,  as  regards  their  powers 
of  absorption.  We  followed  various  molecules  from  a 
state  of  liquid  to  a  state  of  gas,  and  found,  in  both 
states  of  aggregation,  the  power  of  the  individual  mole- 
cules equally  asserted.  The  position  of  a  vapour  as  an 
absorber  of  radiant  heat  was  shown  to  be  determined 
by  that  of  the  liquid  from  which  it  is  derived.  Re- 
versing our  conceptions,  and  regarding  the  molecules 
of  gases  and  vapours  not  as  the  recipients  but  as  the 
originators  of  wave-motion;  not  as  absorbers  but  as 
radiators;  it  was  proved  that  the  powers  of  absorption 
and  radiation  went  hand  in  hand,  the  self-same  chemical 
act  which  rendered  a  body  competent  to  intercept  the 
waves  of  ether,  rendering  it  competent,  in  the  same 
degree,  to  generate  them.  Perfumes  were  next  sub- 
jected to  examination,  and,  notwithstanding  their  ex- 
traordinary tenuity,  they  were  found  vastly  superior, 
in  point  of  absorptive  power,  to  the  body  of  the  air  in 
which  they  were  diffused.  We  were  led  thus  slowly  up 
to  the  examination  of  the  most  widely  diffused  and 
most  important  of  all  vapours — the  aqueous  vapour  of 
our  atmosphere,  and  we  found  in  it  a  potent  absorber 
of  the  purely  calorific  rays.  The  power  of  this  sub- 
stance to  influence  climate,  and  its  general  influence 
on  the  temperature  of  the  earth,  were  then  briefly 
dwelt  upon.  A  cobweb  spread  above  a  blossom  is 


RADIATION.  71 

sufficient  to  protect  it  from  nightly  chill;  and  thus  the 
aqueous  vapour  of  our  air,  attenuated  as  it  is,  checks 
the  drain  of  terrestrial  heat,  and  saves  the  surface  of 
our  planet  from  the  refrigeration  which  would  assuredly 
accrue,  were  no  such  substance  interposed  between  it 
and  the  voids  of  space.  We  considered  the  influence 
of  vibrating  period,  and  molecular  form,  on  absorption 
and  radiation,  and  finally  deduced,  from  its  action 
upon  radiant  heat,  the  exact  amount  of  carbonic  acid 
expired  by  the  human  lungs. 

Thus,  in  brief  outline,  were  placed  before  you  some 
of  the  results  of  recent  enquiries  in  the  domain  of 
Radiation,  and  my  aim  throughout  has  been  to  raise  in 
your  minds  distinct  physical  images  of  the  various  pro- 
cesses involved  in  our  researches.  It  is  thought  by 
some  that  natural  science  has  a  deadening  influence  on 
the  imagination,  and  a  doubt  might  fairly  be  raised  as 
to  the  value  of  any  study  which  would  necessarily  have 
this  effect.  But  the  experience  of  the  last  hour  must, 
I  think,  have  convinced  you,  that  the  study  of  natural 
science  goes  hand  in  hand  with  the  culture  of  the  ima- 
gination. Throughout  the  greater  part  of  this  discourse 
we  have  been  sustained  by  this  faculty.  We  have  been 
picturing  atoms,  and  molecules,  and  vibrations,  and 
waves,  which  eye  has  never  seen  nor  ear  heard,  and 
which  can  only  be  discerned  by  the  exercise  of  ima- 
gination. This,  in  fact,  is  the  faculty  which  enables  us 
to  transcend  the  boundaries  of  sense,  and  connect  the 
phenomena  of  our  visible  world  with  those  of  an  in- 
visible one.  Without  imagination  we  never  could  have 
risen  to  the  conceptions  which  have  occupied  us  here 
to-day;  and  in  proportion  to  your  power  of  exercising 
this  faculty  aright,  and  of  associating  definite  mental 
images  with  the  terms  employed,  will  be  the  pleasure 
and  the  profit  which  you  will  derive  from  this  lecture. 
6 


72  FKAGMENTS    OF    SCIENCE. 

The  outward  facts  of  nature  are  insufficient  to  satisfy 
the  mind.  We  cannot  be  content  with  knowing  that 
the  light  and  heat  of  the  sun  illuminate  and  warm  the 
world.  We  are  led  irresistibly  to  enquire,  'What  is 
light,  and  what  is  heat? '  and  this  question  leads  us  at 
once  out  of  the  region  of  sense  into  that  of  imagination.* 
Thus  pondering,  and  questioning,  and  striving  to 
supplement  that  which  is  felt  and  seen,  but  which  is 
incomplete,  by  something  unfelt  and  unseen  which  is 
necessary  to  its  completeness,  men  of  genius  have  in 
part  discerned,  not  only  the  nature  of  light  and  heat, 
but  also,  through  them,  the  general  relationship  of 
natural  phenomena.  The  working  power  of  Nature 
consists  of  actual  or  potential  motion,  of  which  all  its 
phenomena  are  but  special  forms.  This  motion  mani- 
fests itself  in  tangible  and  in  intangible  matter,  being 
incessantly  transferred  from  the  one  to  the  other,  and 
incessantly  transformed  by  the  change.  It  is  as  real 
in  the  waves  of  the  ether  as  in  the  waves  of  the  sea; 
the  latter — derived  as  they  are  from  winds,  which  in 
their  turn  are  derived  from  the  sun — are,  indeed,  noth- 
ing more  than  the  heaped-up  motion  of  the  ether  waves. 
It  is  the  calorific  waves  emitted  by  the  sun  which  heat 
our  air,  produce  our  winds,  and  hence  agitate  our  ocean. 
And  whether  they  break  in  foam  upon  the  shore,  or 
rub  silently  against  the  ocean's  bed,  or  subside  by  the 
mutual  friction  of  their  own  parts,  the  sea  waves,  which 
cannot  subside  without  producing  heat,  finally  resolve 
themselves  into  waves  of  ether,  thus  regenerating  the 
motion  from  which  their  temporary  existence  was  de- 
rived. This  connection  is  typical.  Nature  is  not  an 
aggregate  of  independent  parts,  but  an  organic  whole. 
If  you  open  a  piano  and  sing  into  it,  a  certain  string 

*  This  line  of  thought  was  pursued  further  five  years  subse- 
quently.   See  '  Scientific  Use  of  the  Imagination '  in  Vol.  II. 


RADIATION.  73 

will  respond.  Change  the  pitch  of  your  voice;  the 
first  string  ceases  to  vibrate,  but  another  replies. 
Change  again  the  pitch;  the  first  two  strings  are  silent, 
while  another  resounds.  Thus  is  sentient  man  acted 
on  by  Nature,  the  optic,  the  auditory,  and  other  nerves 
of  the  human  body  being  so  many  strings  differently 
tuned,  and  responsive  to  different  forms  of  the  universal 
power. 


III. 

ON  RADIANT  HEAT  IN  RELATION  TO  THE 
COLOUR  AND  CHEMICAL  CONSTITUTION  OF 
BODIES* 

ONE  of  the  most  important  functions  of  physical 
'  science,  considered  as  a  discipline  of  the  mind,  is 
to  enable  us  by  means  of  the  sensible  processes  of  Na- 
ture to  apprehend  the  insensible.  The  sensible  pro- 
cesses give  direction  to  the  line  of  thought;  but  this 
once  given,  the  length  of  the  line  is  not  limited  by  the 
boundaries  of  the  senses.  Indeed,  the  domain  of  the 
senses,  in  Nature,  is  almost  infinitely  small  in  com- 
parison with  the  vast  region  accessible  to  thought  which 
lies  beyond  them.  From  a  few  observations  of  a  comet, 
when  it  comes  within  the  range  of  his  telescope,  an  as- 
tronomer can  calculate  its  path  in  regions  which  no 
telescope  can  reach:  and  in  like  manner,  by  means  of 
data  furnished  in  the  narrow  world  of  the  senses,  we 
make  ourselves  at  home  in  other  and  wider  worlds, 
which  are  traversed  by  the  intellect  alone. 

From  the  earliest  ages  the  questions,  '  What  is 
light?'  and  'What  is  heat?'  have  occurred  to  the 
minds  of  men;  but  these  questions  never  would  have 
been  answered  had  they  not  been  preceded  by  the  ques- 
tion, '  What  is  sound?  '  Amid  the  grosser  phenomena 

*  A  discourse  delivered  in  the  Royal  Institution  of  Great 
Britain,  Jan.  19,  1866. 

74 


RADIANT    HEAT    AND   ITS    RELATIONS.         75 

of  acoustics  the  mind  was  first  disciplined,  conceptions 
being  thus  obtained  from  direct  observation,  which 
were  afterwards  applied  to  phenomena  of  a  character 
far  too  subtle  to  be  observed  directly.  Sound  we  know 
to  be  due  to  vibratory  motion.  A  vibrating  tuning- 
fork,  for  example,  moulds  the  air  around  it  into  un- 
dulations or  waves,  which  speed  away  on  all  sides  with 
a  certain  measured  velocity,  impinge  upon  the  drum  of 
the  ear,  shake  the  auditory  nerve,  and  awake  in  the 
brain  the  sensation  of  sound.  When  sufficiently  near 
a  sounding  body  we  can  feel  the  vibrations  of  the  air. 
A  deaf  man,  for  example,  plunging  his  hand  into  a  bell 
when  it  is  sounded,  feels  through  the  common  nerves 
of  his  body  those  tremors  which,  when  imparted  to  the 
nerves  of  healthy  ears,  are  translated  into  sound.  There 
are  various  ways  of  rendering  those  sonorous  vibrations 
not  only  tangible  but  visible;  and  it  was  not  until  num- 
berless experiments  of  this  kind  had  been  executed, 
that  the  scientific  investigator  abandoned  himself 
wholly,  and  without  a  shadow  of  misgiving,  to  the 
conviction  that  what  is  sound  within  us  is,  outside  of 
us,  a  motion  of  the  air. 

But  once  having  established  this  fact — once  having 
proved  beyond  all  doubt  that  the  sensation  of  sound  is 
produced  by  an  agitation  of  the  auditory  nerve — the 
thought  soon  suggested  itself  that  light  might  be  due 
to  an  agitation  of  the  optic  nerve.  This  was  a  great 
step  in  advance  of  that  ancient  notion  which  regarded 
light  as  something  emitted  by  the  eye,  and  not  as  any- 
thing imparted  to  it.  But  if  light  be  produced  by  an 
agitation  of  the  retina,  what  is  it  that  produces  the 
agitation?  Newton,  you  know,  supposed  minute  par- 
ticles to  be  shot  through  the  humours  of  the  eye  against 
the  retina,  which  he  supposed  to  hang  like  a  target  at 
the  back  of  the  eye.  The  impact  of  these  particles 


76  FRAGMENTS    OF    SCIENCE. 

against  the  target,  Newton  believed  to  be  the  cause 
of  light.  But  Newton's  notion  has  not  held  its 
ground,  being  entirely  driven  from  the  field  by  the 
more  wonderful  and  far  more  philosophical  notion  that 
light,  like  sound,  is  a  product  of  wave-motion. 

The  domain  in  which  this  motion  of  light  is  carried 
on  lies  entirely  beyond  the  reach  of  our  senses.  The 
waves  of  light  require  a  medium  for  their  formation 
and  propagation;  but  we  cannot  see,  or  feel,  or  taste, 
or  smell  this  medium.  How,  then,  has  its  existence 
been  established?  By  showing,  that  by  the  assump- 
tion of  this  wonderful  intangible  ether,  all  the  pheno- 
mena of  optics  are  accounted  for,  with  a  fulness,  and 
clearness,  and  conclusiveness,  which  leave  no  desire  of 
the  intellect  unsatisfied.  When  the  law  of  gravitation 
first  suggested  itself  to  the  mind- of  Newton,  what  did 
he  do?  He  set  himself  to  examine  whether  it  accounted 
for  all  the  facts.  He  determined  the  courses  of  the 
planets;  he  calculated  the  rapidity  of  the  moon's  fall 
towards  the  earth;  he  considered  the  procession  of  the 
equinoxes,  the  ebb  and  flow  of  the  tides,  and  found  all 
explained  by  the  law  of  gravitation.  He  therefore  re- 
garded this  law  as  established,  and  the  verdict  of  sci- 
ence subsequently  confirmed  his  conclusion.  On  sim- 
ilar, and,  if  possible,  on  stronger  grounds,  we  found 
our  belief  in  the  existence  of  the  universal  ether.  It 
explains  facts  far  more  various  and  complicated  than 
those  on  which  Newton  based  his  law.  If  a  single 
phenomenon  could  be  pointed  out  which  the  ether  is 
proved  incompetent  to  explain,  we  should  have  to  give 
it  up;  but  no  such  phenomenon  has  ever  been  pointed 
out.  It  is,  therefore,  at  least  as  certain  that  space  is 
filled  with  a  medium,  by  means  of  which  suns  and  stars 
diffuse  their  radiant  power,  as  that  it  is  traversed  by 
that  force  which  holds  in  its  grasp,  not  only  our 


RADIANT    HEAT    AND    ITS    RELATIONS.         77 

planetary  system,  but  the  immeasurable  heavens  them- 
selves. 

There  is  no  more  wonderful  instance  than  this  of 
the  production  of  a  line  of  thought,  from  the  world  of 
the  senses  into  the  region  of  pure  imagination.  I  mean 
by  imagination  here,  not  that  play  of  fancy  which  can 
give  to  airy  nothings  a  local  habitation  and  a  name, 
but  that  power  which  enables  the  mind  to  conceive 
realities  which  lie  beyond  the  range  of  the  senses — to 
present  to  itself  distinct  images  of  processes  which, 
though  mighty  in  the  aggregate  beyond  all  conception, 
are  so  minute  individually  as  to  elude  all  observation. 
It  is  the  waves  of  air  excited  by  a  tuning-fork  which 
render  its  vibrations  audible.  It  is  the  waves  of  ether 
sent  forth  from  those  lamps  overhead  which  render 
them  luminous  to  us;  but  so  minute  are  these  waves, 
that  it  would  take  from  30,000  to  60,000  of  them  placed 
end  to  end  to  cover  a  single  inch.  Their  number,  how- 
ever, compensates  for  their  minuteness.  Trillions  of 
them  have  entered  your  eyes,  and  hit  the  retina  at  the- 
backs  of  your  eyes,  in  the  time  consumed  in  the  utter- 
ance of  the  shortest  sentence  of  this  discourse.  This  is 
the  steadfast  result  of  modern  research;  but  we  never 
could  have  reached  it  without  previous  discipline.  We 
never  could  have  measured  the  waves  of  light,  nor  even 
imagined  them  to  exist,  had  we  not  previously  exercised 
ourselves  among  the  waves  of  sound.  Sound  and  light 
are  now  mutually  helpful,  the  conceptions  of  each  being 
expanded,  strengthened,  and  denned  by  the  conceptions 
of  the  other. 

The  ether  which  conveys  the  pulses  of  light  and 
heat  not  only  fills  celestial  space,  swathing  suns,  and 
planets,  and  moons,  but  it  also  encircles  the  atoms  of 
which  these  bodies  are  composed.  It  is  the  motion  of 
these  atoms,  and  not  that  of  any  sensible  parts  of 


78  FRAGMENTS    OF    SCIENCE. 

bodies,  that  the  ether  conveys.  This  motion  is  the 
objective  cause  of  what,  in  our  sensations,  are  light 
and  heat.  An  atom,  then,  sending  its  pulses  through 
the  ether,  resembles  a  tuning-fork  sending  its  pulses 
through  the  air.  Let  us  look  for  a  moment  at  this 
thrilling  medium,  and  briefly  consider  its  relation  to  the 
bodies  whose  vibrations  it  conveys.  Different  bodies, 
when  heated  to  the  same  temperature,  possess  very  dif- 
ferent powers  of  agitating  the  ether:  some  are  good 
radiators,  others  are  bad  radiators;  which  means  that 
some  are  so  constituted  as  to  communicate  their  atomic 
motion  freely  to  the  ether,  producing  therein  powerful 
undulations;  while  the  atoms  of  others  are  unable  thus 
to  communicate  their  motions,  but  glide  through  the 
medium  without  materially  disturbing  its  repose.  Re- 
cent experiments  have  proved  that  elementary  bodies, 
except  under  certain  anomalous  conditions,  belong  to 
the  class  of  bad  radiators.  An  atom,  vibrating  in  the 
ether,  resembles  a  naked  tuning-fork  vibrating  in  the 
air.  The  amount  of  motion  communicated  to  the  air 
by  the  thin  prongs  is  too  small  to  evoke  at  any  distance 
the  sensation  of  sound.  But  if  we  permit  the  atoms  to 
combine  chemically  and  form  molecules,  the  result,  in 
many  cases,  is  an  enormous  change  in  the  power  of 
radiation.  The  amount  of  ethereal  disturbance,  pro- 
duced by  the  combined  atoms  of  a  body,  may  be  many 
thousand  times  that  produced  by  the  same  atoms  when 
uncombined. 

The  pitch  of  a  musical  note  depends  upon  the 
rapidity  of  its  vibrations,  or,  in  other  words,  on  the 
length  of  its  waves.  Now,  the  pitch  of  a  note  answers 
to  the  colour  of  light.  Taking  a  slice  of  white  light 
from  the  sun,  or  from  an  electric  lamp,  and  causing  the 
light  to  pass  through  an  arrangement  of  prisms,  it  is 
-decomposed.  We  have  the  effect  obtained  by  Newton, 


RADIANT    HEAT    AND    ITS    RELATIONS.         79 

who  first  unrolled  the  solar  beam  into  the  splendours  of 
the  solar  spectrum.  At  one  end  of  this  spectrum  we 
have  red  light,  at  the  other,  violet;  and  between  those 
extremes  lie  the  other  prismatic  colours.  As  we  ad- 
vance along  the  spectrum  from  the  red  to  the  violet,  the 
pitch  of  the  light — if  I  may  use  the  expression — 
heightens,  the  sensation  of  violet  being  produced  by 
a  more  rapid  succession  of  impulses  than  that  which 
produces  the  impression  of  red.  The  vibrations  of  the 
violet  are  about  twice  as  rapid  as  those  of  the  red;  in 
other  words,  the  range  of  the  visible  spectrum  is  about 
an  octave. 

There  is  no  solution  of  continuity  in  this  spectrum; 
one  colour  changes  into  another  by  insensible  grada- 
tions. It  is  as  if  an  infinite  number  of  tuning-forks,  of 
gradually  augmenting  pitch,  were  vibrating  at  the  same 
time.  But  turning  to  another  spectrum — that,  namely, 
obtained  from  the  incandescent  vapour  of  silver — you 
observe  that  it  consists  of  two  narrow  and  intensely 
luminous  green  bands.  Here  it  is  as  if  two  forks  only, 
of  slightly  different  pitch,  were  vibrating.  The  length 
of  the  waves  which  produce  this  first  band  is  such  that 
47,460  of  them,  placed  end  to  end,  would  fill  an  inch. 
The  waves  which  produce  the  second  band  are  a  little 
shorter;  it  would  take  of  these  47,920  to  fill  an  inch. 
In  the  case  of  the  first  band,  the  number  of  impulses 
imparted,  in  one  second,  to  every  eye  which  sees  it,  is 
577  millions  of  millions;  while  the  number  of  impulses 
imparted,  in  the  same  time,  by  the  second  band  is  600 
millions  of  millions.  We  may  project  upon  a  white 
screen  the  beautiful  stream  of  green  light  from  which 
these  bands  were  derived.  This  luminous  stream  is  the 
incandescent  vapour  of  silver.  The  rates  of  vibration 
of  the  atoms  of  that  vapour  are  as  rigidly  fixed  as  those 
of  two  tuning-forks;  and  to  whatever  height  the  tern- 


80  FKAGMENTS    OF    SCIENCE. 

perature  of  the  vapour  may  be  raised,  the  rapidity  of 
its  vibrations,  and  consequently  its  colour,  which  wholly 
depends  upon  that  rapidity,  remain  unchanged. 

The  vapour  of  water,  as  well  as  the  vapour  of  silver, 
has  its  definite  periods  of  vibration,  and  these  are  such 
as  to  disqualify  the  vapour,  when  acting  freely  as  such, 
from  being  raised  to  a  white  heat.  The  oxyhydrogen 
flame,  for  example,  consists  of  hot  aqueous  vapour.  It 
is  scarcely  visible  in  the  air  of  this  room,  and  it  would 
be  still  less  visible  if  we  could  burn  the  gas  in  a  clean 
atmosphere.  But  the  atmosphere,  even  at  the  summit 
of  Mont  Blanc,  is  dirty;  in  London  it  is  more  than 
dirty;  and  the  burning  dirt  gives  to  this  flame  the 
greater  portion  of  its  present  light.  But  the  heat  of 
the  flame  is  enormous.  Cast  iron  fuses  at  a  tempera- 
ture of  2,000°  Fahr.;  while  the  temperature  of  the 
oxyhydrogen  flame  is  6,000°  Fahr.  A  piece  of  platinum 
is  heated  to  vivid  redness,  at  a  distance  of  two  inches 
beyond  the  visible  termination  of  the  flame.  The  va- 
pour which  produces  incandescence  is  here  absolutely 
dark.  In  the  flame  itself  the  platinum  is  raised  to  daz- 
zling whiteness,  and  is  even  pierced  by  the  flame.  When 
this  flame  impinges  on  a  piece  of  lime,  we  have  the 
dazzling  Drummond  light.  But  the  light  is  here  due 
to  the  fact  that  when  it  impinges  upon  the  solid  body, 
the  vibrations  excited  in  that  body  by  the  flame  are  of 
periods  different  from  its  own. 

Thus  far  we  have  fixed  our  attention  on  atoms  and 
molecules  in  a  state  of  vibration,  and  surrounded  by  a 
medium  which  accepts  their  vibrations,  and  transmits 
them  through  space.  But  suppose  the  waves  generated 
by  one  system  of  molecules  to  impinge  upon  another 
system,  how  will  the  waves  be  affected?  Will  they  be 
stopped,  or  will  they  be  permitted  to  pass?  Will  they 
transfer  their  motion  to  the  molecules  on  which  they 


RADIANT    HEAT    AND    ITS    RELATIONS.         81 

impinge,  or  will  they  glide  round  the  molecules,  through 
the  intermolecular  spaces,  and  thus  escape? 

The  answer  to  this  question  depends  upon  a  condi- 
tion which  may  be  beautifully  exemplified  by  an  experi- 
ment on  sound.  These  two  tuning-forks  are  tuned  ab- 
solutely alike.  They  vibrate  with  the  same  rapidity, 
and,  mounted  thus  upon  their  resonant  cases,  you  hear 
them  loudly  sounding  the  same  musical  note.  Stopping 
one  of  the  forks,  I  throw  the  other  into  strong  vibration, 
and  bring  that  other  near  the  silent  fork,  but  not  into 
contact  with  it.  Allowing  them  to  continue  in  this 
position  for  four  or  five  seconds,  and  then  stopping  the 
vibrating  fork,  the  sound  does  not  cease.  The  second 
fork  has  taken  up  the  vibrations  of  its  neighbour,  and 
is  now  sounding  in  its  turn.  Dismounting  one  of  the 
forks,  and  permitting  the  other  to  remain  upon  its 
stand,  I  throw  the  dismounted  fork  into  strong  vibra- 
tion. You  cannot  hear  it  sound.  Detached  from  its 
case,  the  amount  of  motion  which  it  can  communicate 
to  the  air  is  too  small  to  be  sensible  at  any  distance. 
When  the  dismounted  fork  is  brought  close  to  the 
mounted  one,  but  not  into  actual  contact  with  it,  out  of 
the  silence  rises  a  mellow  sound.  Whence  comes  it? 
From  the  vibrations  which  have  been  transferred  from 
the  dismounted  fork  to  the  mounted  one. 

That  the  motion  should  thus  transfer  itself  through 
the  air  it  is  necessary  that  the  two  forks  should  be  in 
perfect  unison.  If  a  morsel  of  wax  not  larger  than  a 
pea  be  placed  on  one  of  the  forks,  it  is  rendered  thereby 
powerless  to  affect,  or  to  be  affected  by,  the  other.  It 
is  easy  to  understand  this  experiment.  The  pulses  of 
the  one  fork  can  affect  the  other,  because  they  are  per- 
fectly timed,  A  single  pulse  causes  the  prong  of  the 
silent  fork  to  vibrate  through  an  infinitesimal  space. 
But  just  as  it  has  completed  this  small  vibration, 


82  FKAGMENTS    OF    SCIENCE. 

another  pulse  is  ready  to  strike  it.  Thus,  the  impulses 
add  themselves  together.  In  the  five  seconds  during 
which  the  forks  were  held  near  each  other,  the  vibrating 
fork  sent  1,280  waves  against  its  neighbour  and  those 
1,280  shocks,  all  delivered  at  the  proper  moment,  all, 
as  I  have  said,  perfectly  timed,  have  given  such  strength 
to  the  vibrations  of  the  mounted  fork  as  to  render  them 
audible  to  all. 

Another  curious  illustration  of  the  influence  of 
synchronism  on  musical  vibrations,  is  this:  Three  small 
gas-flames  are  inserted  into  three  glass  tubes  of  different 
lengths.  Each  of  these  flames  can  be  caused  to  emit 
a  musical  note,  the  pitch  of  which  is  determined  by 
the  length  of  the  tube  surrounding  the  flame.  The 
shorter  the  tube  the  higher  is  the  pitch.  The  flames 
are  now  silent  within  their  respective  tubes,  but  each 
of  them  can  be  caused  to  respond  to  a  proper  note 
sounded  anywhere  in  this  room.  With  an  instrument 
called  a  syren,  a  powerful  musical  note,  of  gradually 
increasing  pitch,  can  be  produced.  Beginning  with  a 
low  note,  and  ascending  gradually  to  a  higher  one,  we 
finally  attain  the  pitch  of  the  flame  in  the  longest  tube. 
The  moment  it  is  reached,  the  flame  bursts  into  song. 
The  other  flames  are  still  silent  within  their  tubes. 
But  by  urging  the  instrument  on  to  higher  notes,  the 
second  flame  is  started,  and  the  third  alone  remains. 
A  still  higher  note  starts  it  also.  Thus,  as  the  sound  of 
the  syren  rises  gradually  in  pitch,  it  awakens  every 
flame  in  passing,  by  striking  it  with  a  series  of  waves 
whose  periods  of  recurrence  are  similar  to  its  own. 

Now  the  wave-motion  from  the  syren  is  in  part  taken 
up  by  the  flame  which  synchronises  with  the  waves;  and 
were  these  waves  to  impinge  upon  a  multitude  of  flames, 
instead  of  upon  one  flame  only,  the  transference  might 
be  so  great  as  to  absorb  the  whole  of  the  original  wave- 


RADIANT    HEAT    AND   ITS    RELATIONS.         83 

motion.  Let  us  apply  these  facts  to  radiant  heat.  This 
blue  flame  is  the  flame  of  carbonic  oxide;  this  trans- 
parent gas  is  carbonic  acid  gas.  In  the  blue  flame  we 
have  carbonic  acid  intensely  heated,  or,  in  other  words, 
in  a  state  of  intense  vibration.  It  thus  resembles  the 
sounding  fork,  while  this  cold  carbonic  acid  resembles 
the  silent  one.  What  is  the  consequence?  Through 
the  synchronism  of  the  hot  and  cold  gas,  the  waves 
emitted  by  the  former  are  intercepted  by  the  latter, 
the  transmission  of  the  radiant  heat  being  thus  pre- 
vented. The  cold  gas  is  intensely  opaque  to  the  radia- 
tion from  this  particular  flame,  though  highly  trans- 
parent to  heat  of  every  other  kind.  We  are  here  mani- 
festly dealing  with  that  great  principle  which  lies  at 
the  basis  of  spectrum  analysis,  and  which  has  enabled 
scientific  men  to  determine  the  substances  of  which  the 
sun,  the  stars,  and  even  the  nebulae  are  composed;  the 
principle,  namely,  that  a  body  which  is  competent  to 
emit  any  ray,  whether  of  heat  or  light,  is  competent  in 
the  same  degree  to  absorb  that  ray.  The  absorption 
depends  On  the  synchronism  existing  between  the  vibra- 
tions of  the  atoms  from  which  .the  rays,  or  more  cor- 
rectly the  waves,  issue,  and  those  of  the  atoms  on  which 
they  impinge. 

To  its  almost  total  incompetence  to  emit  white  light, 
aqueous  vapour  adds  a  similar  incompetence  to  absorb 
white  light.  It  cannot,  for  example,  absorb  the  lumi- 
nous rays  of  the  sun,  though  it  can  absorb  the  non-lumi- 
nous rays  of  the  earth.  This  incompetence  of  the  va- 
pour to  absorb  luminous  rays  is  shared  by  water  and  ice 
— in  fact,  by  all  really  transparent  substances.  Their 
transparency  is  due  to  their  inability  to  absorb  lumi- 
nous rays.  The  molecules  of  such  substances  are  in 
dissonance  with  the  luminous  waves;  and  hence  such 
waves  pass  through  transparent  bodies  without  disturb- 


84  FRAGMENTS    OF    SCIENCE. 

ing  the  molecular  rest.  A  purely  luminous  beam,  how- 
ever intense  may  be  its  heat,  is  sensibly  incompetent  to 
melt  ice.  We  can,  for  example,  converge  a  powerful 
luminous  beam  upon  a  surface  covered  with  hoar  frost, 
without  melting  a  single  spicula  of  the  crystals.  How 
then,  it  may  be  asked,  are  the  snows  of  the  Alps  swept 
away  by  the  sunshine  of  summer?  I  answer,  they  are 
not  swept  away  by  sunshine  at  all,  but  by  rays  which 
have  no  sunshine  whatever  in  them.  The  luminous  rays 
of  the  sun  fall  upon  the  snow-fields  and  are  flashed  in 
echoes  from  crystal  to  crystal,  but  they  find  next  to  no 
lodgment  within  the  crystals.  They  are  hardly  at  all 
absorbed,  and  hence  they  cannot  produce  fusion.  But 
a  body  of  powerful  dark  rays  is  emitted  by  the  sun;  and 
it  is  these  that  cause  the  glaciers  to  shrink  and  the 
snows  to  disappear;  it  is  they  that  fill  the  banks  of  the 
Arve  and  Arveyron,  and  liberate  from  their  frozen  cap- 
tivity the  Rhone  and  the  Ehine. 

Placing  a  concave  silvered  mirror  behind  the  elec- 
tric light  its  rays  are  converged  to  a  focus  of  dazzling 
brilliancy.  Placing  in  the  path  of  the  rays,  between 
the  light  and  the  focus,  a  vessel  of  water,  and  introduc- 
ing at  the  focus  a  piece  of  ice,  the  ice  is  not  melted  by 
the  concentrated  beam.  Matches,  at  the  same  place,  are 
ignited,  and  wood  is  set  on  fire.  The  powerful  heat, 
then,  of  this  luminous  beam  is  incompetent  to  melt  the 
ice.  On  withdrawing  the  cell  of  water,  the  ice  imme- 
diately liquefies,  and  the  water  trickles  from  it  in  drops. 
Reintroducing  the  cell  of  water,  the  fusion  is  arrested, 
and  the  drops  cease  to  fall.  The  transparent  water  of 
the  cell  exerts  no  sensible  absorption  on  the  luminous 
rays,  still  it  withdraws  something  from  the  beam,  which, 
when  permitted  to  act,  is  competent  to  melt  the  ice. 
This  something  is  the  dark  radiation  of  the  electric 
light.  Again,  I  place  a  slab  of  pure  ice  in  front  of  the 


RADIANT    HEAT    AND   ITS    RELATIONS.         85 

electric  lamp;  send  a  luminous  beam  first  through  our 
cell  of  water  and  then  through  the  ice.  By  means  of 
a  lens  an  image  of  the  slab  is  cast  upon  a  white  screen. 
The  beam,  sifted  by  the  water,  has  little  power  upon  the 
ice.  But  observe  what  occurs  when  the  water  is  re- 
moved; we  have  here  a  star  and  there  a  star,  each  star 
resembling  a  flower  of  six  petals,  and  growing  visibly 
larger  before  our  eyes.  As  the  leaves  enlarge,  their 
edges  become  serrated,  but  there  is  no  deviation  from 
the  six-rayed  type.  We  have  here,  in  fact,  the  crystal- 
lisation of  the  ice  reversed  by  the  invisible  rays  of  the 
electric  beam.  They  take  the  molecules  down  in  this 
wonderful  way,  and  reveal  to  us  the  exquisite  atomic 
structure  of  the  substance  with  which  Nature  every 
winter  roofs  our  ponds  and  lakes. 

Numberless  effects,  apparently  anomalous,  might  be 
adduced  in  illustration  of  the  action  of  these  lightless 
rays.  These  two  powders,  for  example,  are  both  white, 
and  undistinguishable  from  each  other  by  the  eye. 
The  luminous  rays  of  the  sun  are  unabsorbed  by  both — 
from  such  rays  these  powders  acquire  no  heat;  still  one 
of  them,  sugar,  is  heated  so  highly  by  the  concentrated 
beam  of  the  electric  lamp,  that  it  first  smokes  and  then 
violently  inflames,  while  the  other  substance,  salt,  is 
barely  warmed  at  the  focus.  Placing  two  perfectly 
transparent  liquids  in  test-tubes  at  the  focus,  one  of 
them  boils  in  a  couple  of  seconds,  while  the  other,  in  a 
similar  position,  is  hardly  warmed.  The  boiling-point 
of  the  first  liquid  is  78°  C.,  which  is  speedily  reached; 
that  of  the  second  liquid  is  only  48°  C.,  which  is  never 
reached  at  all.  These  anomalies  are  entirely  due  to  the 
unseen  element  which  mingles  with  the  luminous  rays 
of  the  electric  beam,  and  indeed  constitutes  90  per  cent, 
of  its  calorific  power. 

A  substance,  as  many  of  you  know,  has  been  dis- 


86  FRAGMENTS    OF    SCIENCE. 

covered,  by  which  these  dark  rays  may  be  detached 
from  the  total  emission  of  the  electric  lamp.  This  ray- 
filter  is  a  liquid,  black  as  pitch  to  the  luminous,  but 
bright  as  a  diamond  to  the  non-luminous,  radiation.  It 
mercilessly  cuts  off  the  former,  but  allows  the  latter 
free  transmission.  When  these  invisible  rays  are 
brought  to  a  focus,  at  a  distance  of  several  feet  from 
the  electric  lamp,  the  dark  rays  form  an  invisible  image 
of  their  source.  By  proper  means,  this  image  may  be 
transformed  into  a  visible  one  of  dazzling  brightness. 
It  might,  moreover,  be  shown,  if  time  permitted,  how, 
out  of  those  perfectly  dark  rays,  could  be  extracted,  by 
a  process  of  transmutation,  all  the  colours  of  the  solar 
spectrum.  It  might  also  be  proved  that  those  rays, 
powerful  as  they  are,  and  sufficient  to  fuse  many  metals, 
can  be  permitted  to  enter  the  eye,  and  to  break  upon 
the  retina,  without  producing  the  least  luminous  im- 
pression. 

The  dark  rays  being  thus  collected,  you  see  nothing 
at  their  place  of  convergence.  With  a  proper  thermo- 
meter it  could  be  proved  that  even  the  air  at  the  focus 
is  just  as  cold  as  the  surrounding  air.  And  mark  the 
conclusion  to  which  this  leads.  It  proves  the  ether  at 
the  focus  to  be  practically  detached  from  the  air, — that 
the  most  violent  ethereal  motion  may  there  exist,  with- 
out the  least  aerial  motion.  But,  though  you  see  it 
not,  there  is  sufficient  heat  at  that  focus  to  set  Lon- 
don on  fire.  The  heat  there  is  competent  to  raise 
iron  to  a  temperature  at  which  it  throws  off  brilliant 
scintillations.  It  can  heat  platinum  to  whiteness,  and 
almost  fuse  that  refractory  metal.  It  actually  can  fuse 
gold,  silver,  copper,  and  aluminium.  The  moment, 
moreover,  that  wood  is  placed  at  the  focus  it  bursts  into 
a  blaze. 

It  has  been  already  affirmed  that,  whether  as  re- 


RADIANT    HEAT    AND    ITS    RELATIONS.         87 

gards  radiation  or  absorption,  the  elementary  atoms 
possess  but  little  power.  This  might  be  illustrated  by 
a  long  array  of  facts;  and  one  of  the  most  singular  of 
these  is  furnished  by  the  deportment  of  that  extremely 
combustible  substance,  phosphorus,  when  placed  at  the 
dark  focus.  It  is  impossible  to  ignite  there  a  fragment 
of  amorphous  phosphorus.  But  ordinary  phosphorus  is 
a  far  quicker  combustible,  and  its  deportment  towards 
radiant  heat  is  still  more  impressive.  It  may  be  ex- 
posed to  the  intense  radiation  of  an  ordinary  fire  with- 
out bursting  into  flame.  It  may  also  be  exposed  for 
twenty  or  thirty  seconds  at  an  obscure  focus,  of  suffi- 
cient power  to  raise  platinum  to  a  red  heat,  without 
ignition.  Notwithstanding  the  energy  of  the  ethereal 
waves  here  concentrated,  notwithstanding  the  extremely 
inflammable  character  of  the  elementary  body  exposed 
to  their  action,  the  atoms  of  that  body  refuse  to  partake 
of  the  motion  of  the  powerful  waves  of  low  refrangi- 
bility,  and  consequently  cannot  be  affected  by  their  heat. 
The  knowledge  we  now  possess  will  enable  us  to 
analyse  with  profit  a  practical  question.  White  dresses 
are  worn  in  summer,  because  they  are  found  to  be 
cooler  than  dark  ones.  The  celebrated  Benjamin 
Franklin  placed  bits  of  cloth  of  various  colours  upon 
snow,  exposed  them  to  direct  sunshine,  and  found  that 
they  sank  to  different  depths  in  the  snow.  The  black 
cloth  sank  deepest,  the  white  did  not  sink  at  all. 
Franklin  inferred  from  this  experiment  that  black 
bodies  are  the  best  absorbers,  and  white  ones  the  worst 
absorbers,  of  radiant  heat.  Let  us  test  the  generality 
of  this  conclusion.  One  of  these  two  cards  is  coated 
with  a  very  dark  powder,  and  the  other  with  a  perfectly 
white  one.  I  place  the  powdered  surfaces  before  a  fire, 
and  leave  them  there  until  they  have  acquired  as  high 
a  temperature  as  they  can  attain  in  this  position. 
7 


88  FKAGMENTS    OF    SCIENCE. 

Which  of  the  cards  is  then  most  highly  heated?  It 
requires  no  thermometer  to  answer  this  question.  Sim- 
plv  pressing  the  back  of  the  card,  on  which  the  white 
powder  is  strewn,  against  the  cheek  or  forehead,  it  is 
found  intolerably  hot.  Placing  the  dark  card  in  the 
same  position,  it  is  found  cool.  The  white  powder  has 
absorbed  far  more  heat  than  the  dark  one.  This  sim- 
ple result  abolishes  a  hundred  conclusions  which  have 
been  hastily  drawn  from  the  experiment  of  Franklin. 
Again,  here  are  suspended  two  delicate  mercurial  ther- 
mometers at  the  same  distance  from  a  gas-flame.  The 
bulb  of  one  of  them  is  covered  by  a  dark  substance,  the 
bulb  of  the  other  by  a  white  one.  Both  bulbs  have  re- 
ceived the  radiation  from  the  flame,  but  the  white  bulb 
has  absorbed  most,  and  its  mercury  stands  much  higher 
than  that  of  the  other  thermometer.  This  experi- 
ment might  be  varied  in  a  hundred  ways:  it  proves 
that  from  the  darkness  of  a  body  you  can  draw  no  cer- 
tain conclusion  regarding  its  power  of  absorption. 

The  reason  of  this  simply  is,  that  colour  gives  us 
intelligence  of  only  one  portion,  and  that  the  smallest 
one,  of  the  rays  impinging  on  the  coloured  body.  Were 
the  rays  all  luminous,  we  might  with  certainty  infer 
from  the  colour  of  a  body  its  power  of  absorption;  but 
the  great  mass  of  the  radiation  from  our  fire,  our  gas- 
flame,  and  even  from  the  sun  itself,  consists  of  invisible 
calorific  rays,  regarding  which  colour  teaches  us  noth- 
ing. A  body  may  be  highly  transparent  to  the  one 
class  of  rays,  and  highly  opaque  to  the  other.  Thus 
the  white  powder,  which  has  shown  itself  so  powerful 
an  absorber,  has  been  specially  selected  on  account  of 
its  extreme  perviousness  to  the  visible  rays,  and  its 
extreme  imperviousness  to  the  invisible  ones;  while  the 
dark  powder  was  chosen  on  account  of  its  extreme 
transparency  to  the  invisible,  and  its  extreme  opacity 


RADIANT    HEAT    AND    ITS    RELATIONS.         89 

to  the  visible,  rays.  In  the  case  of  the  radiation  from 
our  fire,  about  98  per  cent,  of  the  whole  emission  con- 
sists of  invisible  rays;  the  body,  therefore,  which  was 
most  opaque  to  these  triumphed  as  an  absorber,  though 
that  body  was  a  white  one. 

And  here  it  is  worth  while  to  consider  the  manner 
in  which  we  obtain  from  natural  facts  what  may  be 
called  their  intellectual  value.  Throughout  the  pro- 
cesses of  Nature  we  have  interdependence  and  harmony; 
and  the  main  value  of  physics,  considered  as  a  mental 
discipline,  consists  in  the  tracing  out  of  this  interde- 
pendence, and  the  demonstration  of  this  harmony.  The 
outward  and  visible  phenomena  are  the  counters  of  the 
intellect;  and  our  science  would  not  be  worthy  of  its 
name  and  fame  if  it  halted  at  facts,  however  practically 
useful,  and  neglected  the  laws  which  accompany  and 
rule  the  phenomena.  Let  us  endeavour,  then,  to  ex- 
tract from  the  experiment  of  Franklin  all  that  it  can 
yield,  calling  to  our  aid  the  knowledge  which  our  pred- 
ecessors have  already  stored.  Let  us  imagine  two  pieces 
of  cloth  of  the  same  texture,  the  one  black  and  the 
other  white,  placed  upon  sunned  snow.  Fixing  our 
attention  on  the  white  piece,  let  us  enquire  whether 
there  is  any  reason  to  expect  that  it  will  sink  in  the 
snow  at  all.  There  is  knowledge  at  hand  which  enables 
us  to  reply  at  once  in  the  negative.  There  is,  on  the 
contrary,  reason  to  expect  that,  after  a  sufficient  ex- 
posure, the  bit  of  cloth  will  be  found  on  an  eminence 
instead  of  in  a  hollow;  that  instead  of  a  depression, 
we  shall  have  a  relative  elevation  of  the  bit  of  cloth. 
For,  as  regards  the  luminous  rays  of  the  sun,  the  cloth 
and  the  snow  are  alike  powerless;  the  one  cannot  be 
warmed,  nor  the  other  melted,  by  such  rays.  The 
cloth  is  white  and  the  snow  is  white,  because  their 
confusedly  mingled  fibres  and  particles  are  incompetent 


90  FRAGMENTS    OF    SCIENCE. 

to  absorb  the  luminous  rays.  Whether,  then,  the  cloth 
will  sink  or  not  depends  entirely  upon  the  dark  rays 
of  the  sun.  Now  the  substance  which  absorbs  these 
dark  rays  with  the  greatest  avidity  is  ice, — or  snow, 
which  is  merely  ice  in  powder.  Hence,  a  less  amount 
of  heat  will  be  lodged  in  the  cloth  than  in  the  sur- 
rounding snow.  The  cloth  must  therefore  act  as  a 
shield  to  the  snow  on  which  it  rests;  and,  in  conse- 
quence of  the  more  rapid  fusion  of  the  exposed  snow, 
its  shield  must,  in  due  time,  be  left  behind,  perched 
upon  an  eminence  like  a  glacier-table. 

But  though  the  snow  transcends  the  cloth,  both  as  a 
radiator  and  absorber,  it  does  not  much  transcend  it. 
Cloth  is  very  powerful  in  both  these  respects.  Let  us 
now  turn  our  attention  to  the  piece  of  black  cloth,  the 
texture  and  fabric  of  which  I  assume  to  be  the  same  as 
that  of  the  white.  For  our  object  being  to  compare 
the  effects  of  colour,  we  must,  in  order  to  study  this 
effect  in  its  purity,  preserve  all  the  other  conditions 
constant.  Let  us  then  suppose  the  black  cloth  to  be 
obtained  from  the  dyeing  of  the  white.  The  cloth 
itself,  without  reference  to  the  dye,  is  nearly  as  good  an 
absorber  of  heat  as  the  snow  around  it.  But  to  the 
absorption  of  the  dark  solar  rays  by  the  undyed  cloth,  is 
now  added  the  absorption  of  the  whole  of  the  luminous 
rays,  and  this  great  additional  influx  of  heat  is  far  more 
than  sufficient  to  turn  the  balance  in  favour  of  the 
black  cloth.  The  sum  of  its  actions  on  the  dark  and 
luminous  rays,  exceeds  the  action  of  the  snow  on  the 
dark  rays  alone.  Hence  the  cloth  will  sink  in  the  snow, 
and  this  is  the  complete  analysis  of  Frankin's  experi- 
ment. 

Throughout  this  discourse  the  main  stress  has  been 
laid  on  chemical  constitution,  as  influencing  most  pow- 
erfully the  phenomena  of  radiation  and  absorption. 


RADIANT    HEAT   AND   ITS    RELATIONS.         91 

With  regard  to  gases  and  vapours,  and  to  the  liquids 
from  which  these  vapours  are  derived,  it  has  been 
proved  by  the  most  varied  and  conclusive  experiments 
that  the  acts  of  radiation  and  absorption  are  molecular 
— that  they  depend  upon  chemical,  and  not  upon  me- 
chanical, condition.  In  attempting  to  extend  this  prin- 
ciple to  solids  I  was  met  by  a  multitude  of  facts,  ob- 
tained by  celebrated  experimenters,  which  seemed  flatly 
to  forbid  such  an  extension.  Melloni,  for  example, 
had  found  the  same  radiant  and  absorbent  power  for 
chalk  and  lamp-black.  MM.  Masson  and  Courtepee 
had  performed  a  most  elaborate  series  of  experiments  on 
chemical  precipitates  of  various  kinds,  and  found  that 
they  one  and  all  manifested  the  same  power  of  radia- 
tion. They  concluded  from  their  researches,  that  when 
bodies  are  reduced  to  an  extremely  fine  state  of  di- 
vision, the  influence  of  this  state  is  so  powerful  as  en- 
tirely to  mask  and  override  whatever  influence  may  be 
due  to  chemical  constitution. 

But  it  appears  to  me  that  through  the  whole  of  these 
researches  an  oversight  has  run,  the  mere  mention  of 
which  will  show  what  caution  is  essential  in  the  opera- 
tions of  experimental  philosophy;  while  an  experiment 
or  two  will  make  clear  wherein  the  oversight  consists. 
Filling  a  brightly  polished  metal  cube  with  boiling 
water,  I  determine  the  quantity  of  heat  emitted  by  two 
of  the  bright  surfaces.  As  a  radiator  of  heat  one  of 
them  far  transcends  the  other.  Both  surfaces  appear 
to  be  metallic;  what,  then,  is  the  cause  of  the  observed 
difference  in  their  radiative  power?  Simply  this:  one 
of  the  surfaces  is  coated  with  transparent  gum,  through 
which,  of  course,  is  seen  the  metallic  lustre  behind;  and 
this  varnish,  though  so  perfectly  transparent  to  lumi- 
nous rays,  is  as  opaque  as  pitch,  or  lamp-black,  to  non- 
luminous  ones.  It  is  a  powerful  emitter  of  dark  rays; 


92  FEAGMENTS    OF    SCIENCE. 

it  is  also  a  powerful  absorber.  While,  therefore,  at  the 
present  moment,  it  is  copiously  pouring  forth  radiant 
heat  itself,  it  does  not  allow  a  single  ray  from  the  metal 
behind  to  pass  through  it.  The  varnish  then,  and  not 
the  metal,  is  the  real  radiator. 

Now  Melloni,  and  Masson,  and  Courtepee  experi- 
mented thus:  they  mixed  their  powders  and  precipi- 
tates with  gum-water,  and  laid  them,  by  means  of  a 
brush,  upon  the  surfaces  of  a  cube  like  this.  True, 
they  saw  their  red  powders  red,  their  white  ones  white, 
and  their  black  ones  black,  but  they  saw  these  colours 
through  the  coat  of  varnish  which  surrounded  every  par- 
ticle. When,  therefore,  it  was  concluded  that  colour 
had  no  influence  on  radiation,  no  chance  had  been 
given  to  it  of  asserting  its  influence;  when  it  was  found 
that  all  chemical  precipitates  radiated  alike,  it  was  the 
radiation  from  a  varnish,  common  to  them  all,  which 
showed  the  observed  constancy.  Hundreds,  perhaps 
thousands,  of  experiments  on  radiant  heat  have  been 
performed  in  this  way,  by  various  enquirers,  but  the 
work  will,  I  fear,  have  to  be  done  over  again.  I  am  not, 
indeed,  acquainted  with  an  instance  in  which  an  over- 
sight of  so  trivial  a  character  has  been  committed  by  so 
many  able  men  in  succession,  vitiating  so  large  an 
amount  of  otherwise  excellent  work. 

Basing  our  reasonings  thus  on  demonstrated  facts, 
we  arrive  at  the  extremely  probable  conclusion  that 
the  envelope  of  the  particles,  and  not  the  particles 
themselves,  was  the  real  radiator  in  the  experiments 
just  referred  to.  To  reason  thus,  and  deduce  their 
more  or  less  probable  consequences  from  experimental 
facts,  is  an  incessant  exercise  of  the  student  of  physical 
science.  But  having  thus  followed,  for  a  time,  the 
light  of  reason  alone  throTigh  a  series  of  phenomena, 
and  emerged  from  them  with  a  purely  intellectual  con- 


RADIANT    HEAT   AND   ITS    RELATIONS.         93 

elusion,  our  duty  is  to  bring  that  conclusion  to  an  ex- 
perimental test.     In  this  way  we  fortify  our  science. 

For  the  purpose  of  testing  our  conclusion  regarding 
the  influence  of  the  gum,  I  take  two  powders  presenting 
the  same  physical  appearance;  one  of  them  is  a  com- 
pound of  mercury,  and  the  other  a  compound  of  lead. 
On  two  surfaces  of  a  cube  are  spread  these  bright  red 
powders,  without  varnish  of  any  kind.  Filling  the 
cube  with  boiling  water,  and  determining  the  radiation 
from  the  two  surfaces,  one  of  them  is  found  to  emit 
thirty-nine  units  of  heat,  while  the  other  emits  seventy- 
four.  This,  surely,  is  a  great  difference.  Here,  how- 
ever, is  a  second  cube,  having  two  of  its  surfaces  coated 
with  the  same  powders,  the  only  difference  being  that 
the  powders  are  laid  on  by  means  of  a  transparent  gum. 
Both  surfaces  are  now  absolutely  alike  in  radiative 
power.  Both  of  them  emit  somewhat  more  than  was 
emitted  by  either  of  the  unvarnished  powders,  simply 
because  the  gum  employed  is  a  better  radiator  than 
either  of  them.  Excluding  all  varnish,  and  comparing 
white  with  white,  vast  differences  are  found;  comparing 
black  with  black,  they  are  also  different;  and  when 
black  and  white  are  compared,  in  some  cases  the  black 
radiates  far  more  than  the  white,  while  in  other  cases 
the  white  radiates  far  more  than  the  black.  Deter- 
mining, moreover,  the  absorptive  power  of  those  pow- 
ders, it  is  found  to  go  hand-in-hand  with  their  radiative 
power.  The  good  radiator  is  a  good  absorber,  and  the 
bad  radiator  is  a  bad  absorber.  From  all  this  it  is  evi- 
dent that  as  regards  the  radiation  and  absorption  of 
non-luminous  heat,  colour  teaches  us  nothing;  and  that 
even  as  regards  the  radiation  of  the  sun,  consisting  as  it 
does  mainly  of  non-luminous  rays,  conclusions  as  to 
the  influence  of  colour  may  be  altogether  delusive.  This 
is  the  strict  scientific  upshot  of  our  researches.  But 


94  FRAGMENTS    OF    SCIENCE. 

it  is  not  the  less  true  that  in  the  case  of  wearing  ap- 
parel— and  this  for  reasons  which  I  have  given  in 
analysing  the  experiment  of  Franklin — black  dresses 
are  more  potent  than  white  ones  as  absorbers  of  solar 
heat. 

Thus,  in  brief  outline,  have  been  brought  before 
you  a  few  of  the  results  of  recent  enquiry.  If  you  ask 
me  what  is  the  use  of  them,  I  can  hardly  answer  you, 
unless  you  define  the  term  use.  If  you  meant  to  ask 
whether  those  dark  rays  which  clear  away  the  Alpine 
snows,  will  ever  be  applied  to  the  roasting  of  turkeys, 
or  the  driving  of  steam-engines — while  affirming  their 
power  to  do  both,  I  would  frankly  confess  that  they 
are  not  at  present  capable  of  competing  profitably  with 
coal  in  these  particulars.  Still  they  may  have  great 
uses  unknown  to  me;  and  when  our  coal-fields  are  ex- 
hausted, it  is  possible  that  a  more  ethereal  race  than  we 
are  may  cook  their  victuals,  and  perform  their  work, 
in  this  transcendental  way.  But  is  it  necessary  that 
the  student  of  science  should  have  his  labours  tested 
by  their  possible  practical  applications?  What  is  the 
practical  value  of  Homer's  Iliad?  You  smile,  and  pos- 
sibly think  that  Homer's  Iliad  is  good  as  a  means  of 
culture.  There's  the  rub.  The  people  who  demand 
of  science  practical  uses,  forget,  or  do  not  know,  that 
it  also  is  great  as  a  means  of  culture — that  the  knowl- 
edge of  this  wonderful  universe  is  a  thing  profitable  in 
itself,  and  requiring  no  practical  application  to  justify 
its  pursuit. 

But  while  the  student  of  Nature  distinctly  refuses 
to  have  his  labours  judged  by  their  practical  issues,  un- 
less the  term  practical  be  made  to  include  mental  as 
well  as  material  good,  he  knows  full  well  that  the 
greatest  practical  triumphs  have  been  episodes  in  the 
search  after  pure  natural  truth.  The  electric  telegraph 


RADIANT   HEAT   AND   ITS   RELATIONS.         95 

is  the  standing  wonder  of  this  age,  and  the  men  whose 
scientific  knowledge,  and  mechanical  skill,  have  made 
the  telegraph  what  it  is,  are  deserving  of  all  honour. 
In  fact,  they  have  had  their  reward,  both  in  reputation 
and  in  those  more  substantial  benefits  which  the  direct 
service  of  the  public  always  carries  in  its  train.  But 
who,  I  would  ask,  put  the  soul  into  this  telegraphic 
body?  AY  ho  snatched  from  heaven  the  fire  that  flashes 
along  the  line?  This,  I  am  bound,  to  say,  was  done  by 
two  men,  the  one  a  dweller  in  Italy,*  the  other  a  dweller 
in  England,!  wno  never  in  their  enquiries  consciously 
set  a  practical  object  before  them — whose  only  stimu- 
lus was  the  fascination  which  draws  the  climber  to  a 
never-trodden  peak,  and  would  have  made  Caesar  quit 
his  victories  for  the  sources  of  the  Nile.  That  the 
knowledge  brought  to  us  by  those  prophets,  priests, 
and  kings  of  science  is  what  the  world  calls  'useful 
knowledge,'  the  triumphant  application  of  their  discov- 
eries proves.  But  science  has  another  function  to  ful- 
fil, in  the  storing  and  the  training  of  the  human  mind; 
and  I  would  base  my  appeal  to  you  on  the  specimen 
which  has  this  evening  been  brought  before  you, 
whether  any  system  of  education  at  the  present  day 
can  be  deemed  even  approximately  complete,  in  which 
the  knowledge  of  Nature  is  neglected  or  ignored. 

*  Volta.  f  Faraday. 


IV. 

NEW  CHEMICAL  REACTIONS  PRODUCED  BY 
LIGHT. 

1868-69. 

MEASUEED  by  their  power,  not  to  excite  vision, 
but  to  produce  heat — in  other  words,  measured 
by  their  absolute  energy — the  ultra-red  waves  of  the 
sun  and  of  the  electric  light,  as  shown  in  the  preceding 
articles,  far  transcend  the  visible.  In  the  domain  of 
chemistry,  however,  there  are  numerous  cases  in  which 
the  more  powerful  waves  are  ineffectual,  while  the  more 
minute  waves,  through  what  may  be  called  their  timeli- 
ness of  application,  are  able  to  produce  great  effects.  A 
series  of  these,  of  a  novel  and  beautiful  character,  dis- 
covered in  1868,  and  further  illustrated  in  subsequent 
years,  may  be  exhibited  by  subjecting  the  vapours  of 
volatile  liquids  to  the  action  of  concentrated  sunlight, 
or  to  the  concentrated  beam  of  the  electric  light.  Their 
investigation  led  up  to  the  discourse  on  '  Dust  and 
Disease '  which  follows  in  this  volume;  and  for  this 
reason  some  account  of  them  is  introduced  here. 

A  glass  tube  3  feet  long  and  3  inches  wide,  which 
had  been  frequently  employed  in  my  researches  on 
radiant  heat,  was  supported  horizontally  on  two  stands. 
At  one  end  of  the  tube  was  placed  an  electric  lamp, 
the  height  and  position  of  both  being  so  arranged,  that 
the  axis  of  the  tube,  and  that  of  the  beam  issuing  from 
96 


DECOMPOSITION    BY    LIGHT.  97 

the  lamp,  were  coincident.  In  the  first  experiments 
the  two  ends  of  the  tube  were  closed  by  plates  of  rock- 
salt,  and  subsequently  by  plates  of  glass.  For  the  sake 
of  distinction,  I  call  this  tube  the  experimental  tube. 
It  was  connected  with  an  air-pump,  and  also  with  a 
series  of  drying  and  other  tubes  used  for  the  purifica- 
tion of  the  air. 

A  number  of  test-tubes,  like  F,  fig.  2  (I  have  used 
at  least  fifty  of  them),  were  converted  into  Woulf's 
flasks.  Each  of  them  was  stopped 
by  a  cork,  through  which  passed 
two  glass  tubes:  one  of  these 
tubes  (a)  ended  immediately 
below  the  cork,  while  the  other 
(b)  descended  to  the  bottom  of 
the  flask,  being  drawn  out  at  its 
lower  end  to  an  orifice  about 
0.03  of  an  inch  in  diameter.  It 
was  found  necessary  to  coat  the 
cork  carefully  with  cement.  In 
the  later  experiments  corks  of 
vulcanised  india-rubber  were  in- 
variably employed. 

The  little  flask,  thus  formed, 
being  partially  filled  with  the 
liquid  whose  vapour  was  to  be 
examined,  was  introduced  into 
the  path  of  the  purified  current 
of  air.  The  experimental  tube 
being  exhausted,  and  the  cock 
which  cut  off  the  supply  of 
purified  air  being  cautiously 
turned  on,  the  air  entered  the  flask  through  the  tube  ft, 
and  escaped  by  the  small  orifice  at  the  lower  end  of 
6  into  the  liquid.  Through  this  it  bubbled,  loading 


98 


FRAGMENTS    OF    SCIENCE. 


DECOMPOSITION    BY    LIGHT.  99 

itself  with  vapour,  after  which  the  mixed  air  and  va- 
pour, passing  from  the  flask  by  the  tube  a,  entered  the 
experimental  tube,  where  they  were  subjected  to  the 
action  of  light. 

The  whole  arrangement  is  shown  in  fig.  3,  where  L 
represents  the  electric  lamp,  s  s'  the  experimental  tube, 
p  p'  the  pipe  leading  to  the  air-pump,  and  F  the  test- 
tube  containing  the  volatile  liquid.  The  tube  1 1'  is 
plugged  with  cotton-wool  intended  to  intercept  the 
floating  matter  of  the  air;  the  bent  tube  T'  contains 
caustic  potash,  the  tube  T  sulphuric  acid,  the  one  in- 
tended to  remove  the  carbonic  acid  and  the  other  the 
aqueous  vapour  of  the  air. 

The  power  of  the  electric  beam  to  reveal  the  ex- 
istence of  anything  within  the  experimental  tube,  or 
the  impurities  of  the  tube  itself,  is  extraordinary. 
When  the  experiment  is  made  in  a  darkened  room,  a 
tube  which  in  ordinary  daylight  appears  absolutely 
clean,  is  often  shown  by  the  present  mode  of  examina- 
tion to  be  exceedingly  filthy. 

The  following  are  some  of  the  results  obtained  with 
this  arrangement: — 

Nitrite  of  amyl. — The  vapour  of  this  liquid  was  in 
the  first  instance  permitted  to  enter  the  experimental 
tube,  while  the  beam  from  the  electric  lamp  was  pass- 
ing through  it.  Curious  clouds,  the  cause  of  which 
was  then  unknown,  were  observed  to  form  near  the 
place  of  entry,  being  afterwards  whirled  through  the 
tube. 

The  tube  being  again  exhausted,  the  mixed  air  and 
vapour  were  allowed  to  enter  it  in  the  dark.  The 
slightly  convergent  beam  of  the  electric  light  was  then 
sent  through  the  mixture.  For  a  moment  the  tube 
was  optically  empty,  nothing  whatever  being  seen 
within  it;  but  before  a  second  had  elapsed  a  shower  of 


100  FRAGMENTS    OF    SCIENCE. 

particles  was  precipitated  on  the  beam.  The  cloud 
thus  generated  became  denser  as  the  light  continued 
to  act,  showing  at  some  places  vivid  iridescence. 

The  lens  of  the  electric  lamp  was  now  placed  so  as 
to  form  within  the  tube  a  strongly  convergent  cone  of 
rays.  The  tube  was  cleansed  and  again  filled  in  dark- 
ness. When  the  light  was  sent  through  it,  the  pre- 
cipitation upon  the  beam  was  so  rapid  and  intense  that 
the  cone,  which  a  moment  before  was- invisible,  flashed 
suddenly  forth  like  a  solid  luminous  spear.  The  effect 
was  the  same  when  the  air  and  vapour  were  allowed  to 
enter  the  tube  in  diffuse  daylight.  The  cloud,  however, 
which  shone  with  such  extraordinary  radiance  under 
the  electric  beam,  was  invisible  in  the  ordinary  light  of 
the  laboratory. 

The  quantity  of  mixed  air  and  vapour  within  the 
experimental  tube  could  of  course  be  regulated  at 
pleasure.  The  rapidity  of  the  action  diminished  with 
the  attenuation  of  the  vapour.  When,  for  example, 
the  mercurial  column  associated  with  the  experimental 
tube  was  depressed  only  five  inches,  the  action  was  not 
nearly  so  rapid  as  when  the  tube  was  full.  In  such 
cases,  however,  it  was  exceedingly  interesting  to  ob- 
serve, after  some  seconds  of  waiting,  a  thin  streamer  of 
delicate  bluish-white  cloud  slowly  forming  along  the 
axis  of  the  tube,  and  finally  swelling  so  as  to  fill  it. 

When  dry  oxygen  was  employed  to  carry  in  the 
vapour,  the  effect  was  the  same  as  that  obtained  with 
air. 

When  dry  hydrogen  was  used  as  a  vehicle,  the  ef- 
fect was  also  the  same. 

The  effect,  therefore,  is  not  due  to  any  interaction 
between  the  vapour  of  the  nitrite  and  its  vehicle. 

This  was  further  demonstrated  by  the  deportment 
of  the  vapour  itself.  When  it  was  permitted  to  enter 


DECOMPOSITION    BY    LIGHT.  101 

the  experimental  tube  unmixed  with  air  or  any  other 
gas,  the  effect  was  substantially  the  same.  Hence  the 
seat  of  the  observed  action  is  the  vapour. 

This  action  is  not  to  be  ascribed  to  heat.  As  re- 
gards the  glass  of  the  experimental  tube,  and  the  air 
within  the  tube,  the  beam  employed  in  these  experi- 
ments was  perfectly  cold.  It  had  been  sifted  by  pass- 
ing it  through  a  solution  of  alum,  and  through  the 
thick  double-convex  lens  of  the  lamp.  When  the  un- 
sifted beam  of  the  lamp  was  employed,  the  effect  was 
still  the  same;  the  obscure  calorific  rays  did  not  appear 
to  interfere  with  the  result. 

My  object  here  being  simply  to  point  out  to  chem- 
ists a  method  of  experiment  which  reveals  a  new  and 
beautiful  series  of  reactions,  I  left  to  them  the  exam- 
ination of  the  products  of  decomposition.  The  group 
of  atoms  forming  the  molecule  of  nitrite  of  amyl  is 
obviously  shaken  asunder  by  certain  specific  waves  of 
the  electric  beam,  nitric  oxide  and  other  products,  of 
which  the  nitrate  of  amyl  is  probably  one,  being  the 
result  of  the  decomposition.  The  brown  fumes  of 
nitrous  acid  were  seen  mingling  with  the  cloud  within 
the  experimental  tube.  The  nitrate  of  amyl,  being 
less  volatile  than  the  nitrite,  and  not  being  able  to 
maintain  itself  in  the  condition  of  vapour,  would  be 
precipitated  as  a  visible  cloud  along  the  track  of  the 
beam. 

In  the  anterior  portions  of  the  tube  a  powerful  sift- 
ing of  the  beam  by  the  vapour  occurs,  which  diminishes 
the  chemical  action  in  the  posterior  portions.  In  some 
experiments  the  precipitated  cloud  only  extended  half- 
way down  the  tube.  When,  under  these  circumstances, 
the  lamp  was  shifted  so  as  to  send  the  beam  through 
the  other  end  of  the  tube,  copious  precipitation  oc- 
curred there  also. 


102  FRAGMENTS    OF    SCIENCE. 

Solar  light  also  effects  the  decomposition  of  the 
nitrite-of-amyl  vapour.  On  October  10,  1868,  I  par- 
tially darkened  a  small  room  in  the  Eoyal  Institution, 
into  which  the  sun  shone,  permitting  the  light  to  enter 
through  an  open  portion  of  the  window-shutter.  In 
the  track  of  the  beam  was  placed  a  large  plano-convex 
lens,  which  formed  a  fine  convergent  cone  in  the  dust 
of  the  room  behind  it.  The  experimental  tube  was 
filled  in  the  laboratory,  covered  with  a  black  cloth,  and 
carried  into  the  partially  darkened  room.  On  thrust- 
ing one  end  of  the  tube  into  the  cone  of  rays  behind  the 
lens,  precipitation  within  the  cone  was  copious  and  im- 
mediate. The  vapour  at  the  distant  end  of  the  tube 
was  in  part  shielded  by  that  in  front,  and  was  also  more 
feebly  acted  on  through  the  divergence  of  the  rays.  On 
reversing  the  tube,  a  second  and  similar  cone  was  pre- 
cipitated. 

Physical  Considerations. 

I  sought  to  determine  the  particular  portion  of  the 
light  which  produced  the  foregoing  effects.  When, 
previous  to  entering  the  experimental  tube,  the  beam 
was  caused  to  pass  through  a  red  glass,  the  effect  was 
greatly  weakened,  but  not  extinguished.  This  was  also 
the  case  with  various  samples  of  yellow  glass.  A  blue 
glass  being  introduced  before  the  removal  of  the  yellow 
or  the  red,  on  taking  the  latter  away  prompt  precipita- 
tion occurred  along  the  track  of  the  blue  beam.  Hence, 
in  this  case,  the  more  refrangible  rays  are  the  most 
chemically  active.  The  colour  of  the  liquid  nitrite  of 
amyl  indicates  that  this  must  be  the  case;  it  is  a  feeble 
but  distinct  yellow:  in  other  words,  the  yellow  portion 
of  the  beam  is  most  freely  transmitted.  It  is  not, 
however,  the  transmitted  portion  of  any  beam  which 
produces  chemical  action,  but  the  absorbed  portion. 


DECOMPOSITION    BY   LIGHT.  _  103 

Blue,  as  the  complementary  colour  to  yellow,  is  here 
absorbed,  and  hence  the  more  energetic  action  of  the 
blue  rays. 

This  reasoning,  however,  assumes  that  the  same  rays 
are  absorbed  by  the  liquid  and  its  vapour.  The  as- 
sumption is  worth  testing.  A  solution  of  the  yellow 
chromate  of  potash,  the  colour  of  which  may  be  made 
almost,  if  not  altogether,  identical  with  that  of  the 
liquid  nitrite  of  amyl,  was  found  far  more  effective  in 
stopping  the  chemical  rays  than  either  the  red  or  the 
yellow  glass.  But  of  all  substances  the  liquid  nitrite 
itself  is  most  potent  in  arresting  the  rays  which  act 
upon  its  vapour.  A  layer  one-eighth  of  an  inch  in 
thickness,  which  scarcely  perceptibly  affected  the 
luminous  intensity,  absorbed  the  entire  chemical  en- 
ergy of  the  concentrated  beam  of  the  electric  light. 

The  close  relation  subsisting  between  a  liquid  and 
its  vapour,  as  regards  their  action  upon  radiant  heat, 
has  been  already  amply  demonstrated.*  As  regards  the 
nitrite  of  amyl,  this  relation  is  more  specific  than  in  the 
cases  hitherto  adduced;  for  here  the  special  constituent 
of  the  beam,  which  provokes  the  decomposition  of  the 
vapour,  is  shown  to  be  arrested  by  the  liquid. 

A  question  of  extreme  importance  in  molecular 
physics  here  arises:  What  is  the  real  mechanism  of  this 
absorption,  and  where  is  its  seat?  f  I  figure,  as  others 
do,  a  molecule  as  a  group  of  atoms,  held  together  by 
their  mutual  forces,  but  still  capable  of  motion  among 
themselves.  The  vapour  of  the  nitrite  of  amyl  is  to 
be  regarded  as  an  assemblage  of  such  molecules.  The 

•'Phil.  Trans.'  1864;  'Heat,  a  Mode  of  Motion,'  chap,  xii.; 
and  p.  61  of  this  volume. 

f  My  attention  was  very  forcibly  directed  to  this  subject  some 
years  ago  by  a  conversation  with  my  excellent  friend  Professor 
Clausius. 


104  „  FRAGMENTS    OF    SCIENCE. 

question  now  before  us  is  this:  In  the  act  of  absorption, 
is  it  the  molecules  that  are  effective,  or  is  it  their  con- 
stituent atoms?  Is  .the  vis  viva  of  the  intercepted 
light-waves  transferred  to  the  molecule  as  a  whole,  or 
to  its  constituent  parts? 

The  molecule,  as  a  whole,  can  only  vibrate  in  virtue 
of  the  forces  exerted  between  it  and  its  neighbour  mole- 
cules. The  intensity  of  these  forces,  and  consequently 
the  rate  of  vibration,  would,  in  this  case,  be  a  function 
of  the  distance  between  the  molecules.  Now  the  iden- 
tical absorption  of  the  liquid  and  of  the  vaporous  nitrite 
of  amyl  indicates  an  identical  vibrating  period  on  the 
part  of  liquid  and  vapour,  and  this,  to  my  mind, 
amounts  to  an  experimental  proof  that  the  absorption 
occurs  in  the  main  within  the  molecule.  Tor  it  can 
hardly  be  supposed,  if  the  absorption  were  the  act  of 
the  molecule  as  a  whole,  that  it  could  continue  to  affect 
waves  of  the  same  period  after  the  substance  had  passed 
from  the  vaporous  to  the  liquid  state. 

In  point  of  fact,  the  decomposition  of  the  nitrite  of 
amyl  is  itself  to  some  extent  an  illustration  of  this  in- 
ternal molecular  absorption;  for  were  the  absorption 
the  act  of  the  molecule  as  a  whole,  the  relative  motions 
of  its  constituent  atoms  would  remain  unchanged,  and 
there  would  be  no  mechanical  cause  for  their  separation. 
It  is  probably  the  synchronism  of  the  vibrations  of  one 
portion  of  the  molecule  with  the  incident  waves,  that 
enables  the  amplitude  of  those  vibrations  to  augment, 
until  the  chain  which  binds  the  parts  of  the  molecule 
together  is  snapped  asunder. 

I  anticipate  wide,  if  not  entire,  generality  for  the 
fact  that  a  liquid  and  its  vapour  absorb  the  same  rays. 
A  cell  of  liquid  chlorine  would,  I  imagine,  deprive  light 
more  effectually  of  its  power  of  causing  chlorine  and 
hydrogen  to  combine  than  any  other  filter  of  the 


DECOMPOSITION    BY   LIGHT.  105 

luminous  rays.  The  rays  which  give  chlorine  its  colour 
have  nothing  to  do  with  this  combination,  those  that 
are  absorbed  by  the  chlorine  being  the  really  effective 
rays.  A  highly  sensitive  bulb,  containing  chlorine  and 
hydrogen,  in  the  exact  proportions  necessary  for  the 
formation  of  hydrochloric  acid,  was  placed  at  one  end  of 
an  experimental  tube,  the  beam  of  the  electric  lamp 
being  sent  through  it  from  the  other.  The  bulb  did 
not  explode  when  the  tube  was  filled  with  chlorine, 
while  the  explosion  was  violent  and  immediate  when 
the  tube  was  filled  with  air.  I  anticipate  for  the  liquid 
chlorine  an  action  similar  to,  but  still  more  energetic 
than,  that  exhibited  by  the  gas.  If  this  should  prove 
to  be  the  case,  it  will  favour  the  view  that  chlorine 
itself  is  molecular  and  not  monatomic. 

Production  of  Sky-blue  by  the  Decomposition  of  Nitrite 
of  Amyl. 

When  the  quantity  of  nitrite  vapour  is  considerable, 
and  the  light  intense,  the  chemical  action  is  exceedingly 
rapid,  the  particles  precipitated  being  so  large  as  to 
whiten  the  luminous  beam.  Not  so,  however,  when  a 
well-mixed  and  highly  attenuated  vapour  fills  the  ex- 
perimental tube.  The  effect  now  to  be  described  was 
first  obtained  when  the  vapour  of  the  nitrite  was  de- 
rived from  a  portion  of  its  liquid  which  had  been  ac- 
cidentally introduced  into  the  passage  through  which 
the  dry  air  flowed  into  the  experimental  tube. 

In  this  case,  the  electric  beam  traversed  the  tube  for 
several  seconds  before  any  action  was  visible.  Decom- 
position then  visibly  commenced,  and  advanced  slowly. 
When  the  light  was  very  strong,  the  cloud  appeared  of 
a  milky  blue.  When,  on  the  contrary,  the  intensity 
was  moderate,  the  blue  was  pure  and  deep.  In  Briicke's 
important  experiments  on  the  blue  of  the  sky  and  the 


106  FRAGMENTS    OF    SCIENCE. 

morning  and  evening  red,  pure  mastic  is  dissolved  in 
alcohol,  and  then  dropped  into  water  well  stirred. 
When  the  proportion  .of  mastic  to  alcohol  is  correct,  the 
resin  is  precipitated  so  finely  as  to  elude  the  highest 
microscopic  power.  By  reflected  light,  such  a  medium 
appears  bluish,  by  transmitted  light  yellowish,  which 
latter  colour,  by  augmenting  the  quantity  of  the  pre- 
cipitate, can  be  caused  to  pass  into  orange  or  red. 

But  the  development  of  colour  in  the  attenuated 
nitrite-of-amyl  vapour  is  doubtless  more  similar  to  what 
takes  place  in  our  atmosphere.  The  blue,  moreover, 
is  far  purer  and  more  sky-like  than  that  obtained  from 
Briicke's  turbid  medium.  Never,  even  in  the  skies  of 
the  Alps,  have  I  seen  a  richer  or  a  purer  blue  than  that 
attainable  by  a  suitable  disposition  of  the  light  falling 
upon  the  precipitated  vapour. 

Iodide  of  Allyl. — Among  the  liquids  hitherto  sub- 
jected to  the  concentrated  electric  light,  iodide  of  allyl, 
in  point  of  rapidity  and  intensity  of  action,  comes  next 
to  the  nitrite  of  amyl.  With  the  iodide  I  have  em- 
ployed both  oxygen  and  hydrogen,  as  well  as  air,  as  a 
vehicle,  and  found  the  effect  in  all  cases  substantially 
the  same.  The  cloud-column  here  was  exquisitely 
beautiful.  It  revolved  round  the  axis  of  the  decom- 
posing beam;  it  was  nipped  at  certain  places  like  an 
hour-glass,  and  round  the  two  bells  of  the  glass  delicate 
cloud-filaments  twisted  themselves  in  spirals.  It  also 
folded  itself  into  convolutions  resembling  those  of 
shells.  In  certain  conditions  of  the  atmosphere  in  the 
Alps  I  have  often  observed  clouds  of  a  special  pearly 
lustre;  when  hydrogen  was  made  the  vehicle  of  the 
iodide-of-allyl  vapour  a  similar  lustre  was  most  ex- 
quisitely shown.  With  a  suitable  disposition  of  the 
light,  the  purple  hue  of  iodine-vapour  came  out  very 
strongly  in  the  tube. 


DECOMPOSITION    BY    LIGHT.  107 

The  remark  already  made,  as  to  the  bearing  of  the 
decomposition  of  nitrite  of  amyl  by  light  on  the  ques- 
tion of  molecular  absorption,  applies  here  also;  for  were 
the  absorption  the  work  of  the  molecule  as  a  whole, 
the  iodine  would  not  be  dislodged  from  the  allyl  with 
which  it  is  combined.  The  non-synchronism  of  iodine 
with  the  waves  of  obscure  heat  is  illustrated  by  its  mar- 
vellous transparency  to  such  heat.  May  not  its  syn- 
chronism with  the  waves  of  light  in  the  present  instance 
be  the  cause  of  its  divorce  from  the  allyl? 

Iodide  of  Isopropyl. — The  action  of  light  upon-  the 
\apour  of  this  liquid  is,  at  first,  more  languid  than  upon 
iodide  of  allyl;  indeed  many  beautiful  reactions  may 
be  overlooked,  in  consequence  of  this  languor  at  the 
commencement.  After  some  minutes'  exposure,  how- 
ever, clouds  begin  to  form,  which  grow  in  density  and 
in  beauty  as  the  light  continues  to  act.  In  every  ex- 
periment hitherto  made  with  this  substance  the  column 
of  cloud  filling  the  experimental  tube,  was  divided  into 
two  distinct  parts  near  the  middle  of  the  tube.  In  one 
experiment  a  globe  of  cloud  formed  at  the  centre,  from 
which,  right  and  left,  issued  an  axis  uniting  the  globe 
with  two  adjacent  cylinders.  Both  globe  and  cylinders 
were  animated  by  a  common  motion  of  rotation.  As 
the  action  continued,  paroxysms  of  motion  were  mani- 
fested; the  various  parts  of  the  clouds  would  rush 
through  each  other  with  sudden  violence.  During 
these  motions  beautiful  and  grotesque  cloud-forms  were 
developed.  At  some  places  the  nebulous  mass  would 
become  ribbed  so  as  to  resemble  the  graining  of  wood;  a 
longitudinal  motion  would  at  times  generate  in  it  a 
series  of  curved  transverse  bands,  the  retarding  influ- 
ence of  the  sides  of  the  tube  causing  an  appearance  re- 
sembling, on  a  small  scale,  the  dirt-bands  of  the  Mer  de 
Glace.  In  the  anterior  portion  of  the  tube  those  sudden 


108  FKAGMENTS    OF    SCIENCE. 

commotions  were  most  intense;  here  buds  of  cloud 
would  sprout  forth,  and  grow  in  a  few  seconds  into  per- 
fect flower-like  forms.  The  cloud  of  iodide  of  isopropyl 
had  a  character  of  its  own,  and  differed  materially  from 
all  others  that  I  had  seen.  A  gorgeous  mauve  colour  was 
observed  in  the  last  twelve  inches  of  the  tube;  the  va- 
pour of  iodine  was  present,  and  it  may  have  been  the 
sky-blue  scattered  by  the  precipitated  particles  which, 
mingling  with  the  purple  of  the  iodine,  produced  the 
mauve.  As  in  all  other  cases  here  adduced,  the  effects 
were  proved  to  be  due  to  the  light;  they  never  occurred 
in  darkness. 

The  forms  assumed  by  some  of  those  actinic  clouds, 
as  I  propose  to  call  them,  in  consequence  of  rotations 
and  other  motions,  due  to  differences  of  temperature, 
are  perfectly  astounding.  I  content  myself  here  with  a 
meagre  description  of  one  more  of  them. 

The  tube  being  filled  with  the  sensitive  mixture,  the 
beam  was  sent  through  it,  the  lens  at  the  same  time 
being  so  placed  as  to  produce  a  cone  of  very  intense 
light.  Two  minutes  elapsed  before  anything  was  vis- 
ible; but  at  the  end  of  this  time  a  faint  bluish  cloud 
appeared  to  hang  itself  on  the  most  concentrated  por- 
tion of  the  beam. 

Soon  afterwards  a  second  cloud  was  formed  five 
inches  farther  down  the  experimental  tube.  Both 
clouds  were  united  by  a  slender  cord  of  the  same  bluish 
tint  as  themselves. 

As  the  action  of  the  light  continued,  the  first  cloud 
gradually  resolved  itself  into  a  series  of  parallel  disks  of 
exquisite  delicacy,  which  rotated  round  an  axis  perpen- 
dicular to  their  surfaces,  and  finally  blended  to  a  screw 
surface  with  an  inclined  generatrix.  This  gradually 
changed  into  a  filmy  funnel,  from  the  narrow  end  of 
which  the  '  cord '  extended  to  the  cloud  in  advance. 


ARTIFICIAL   SKY.  109 

The  latter  also  underwent  slow  but  incessant  modifica- 
tion. It  first  resolved  itself  into  a  series  of  strata  re- 
sembling those  of  the  electric  discharge.  After  a  little 
time,  and  through  changes  which  it  was  difficult  to  fol- 
low, both  clouds  presented  the  appearance  of  a  series  of 
concentric  funnels  set  one  within  the  other,  the  interior 
ones  being  seen  through  the  outer  ones.  Those  of  the 
distant  cloud  resembled  claret-glasses  in  shape.  As 
many  as  six  funnels  were  thus  concentrically  set  to- 
gether, the  two  series  being  united  by  the  delicate  cord 
of  cloud  already  referred  to.  Other  cords  and  slender 
tubes  were  afterwards  formed,  which  coiled  themselves 
in  delicate  spirals  around  the  funnels. 

Rendering  the  light  along  the  connecting-cord  more 
intense,  it  diminished  in  thickness  and  became  whiter; 
this  was  a  consequence  of  the  enlargement  of  its  par- 
ticles. The  cord  finally  disappeared,  while  the  funnels 
melted  into  two  ghost-like  films,  shaped  like  parasols. 
They  were  barely  visible,  being  of  an  exceedingly  deli- 
cate blue  tint.  They  seemed  woven  of  blue  air.  To  com- 
pare them  with  cobweb  or  with  gauze  would  be  to  liken 
them  to  something  infinitely  grosser  than  themselves. 

In  all  cases  a  distant  candle-flame,  when  looked  at 
through  the  cloud,  was  sensibly  undimmed. 


§  2.  ON  THE  BLUE  COLOUR  OF  THE  SKY,  AND  THE 
POLARISATION  OF  SKY  LIGHT.* 

1869. 

After  the  communication  to  the  Royal  Society  of 
the  foregoing  brief  account  of  a  new  Series  of  Chemical 
Reactions  produced  by  Ljght,  the  experiments  upon 

*  In  my  '  Lectures  on  Light '  (Longmans),  the  polarisation  of 
light  will  be  found  briefly,  but,  I  trust,  clearly  explained. 


HO  FEAGMENTS    OF    SCIENCE. 

this  subject  were  continued,  the  number  of  substances 
thus  acted  on  being  considerably  increased. 

I  now,  however,  beg  to  direct  attention  to  two  ques- 
tions glanced  at  incidentally  in  the  preceding  pages — 
the  blue  colour  of  the  sky,  and  the  polarization  of  sky 
light.  Reserving  the  historic  treatment  of  the  subject 
for  a  more  fitting  occasion,  I  would  merely  mention 
now  that  these  questions  constitute,  in  the  opinion  of 
our  most  eminent  authorities,  the  two  great  standing 
enigmas  of  meteorology.  Indeed  it  was  the  interest 
manifested  in  them  by  Sir  John  Herschel,  in  a  letter  of 
singular  speculative  power,  addressed  to  myself,  that 
caused  me  to  enter  upon  the  consideration  of  these 
questions  so  soon. 

The  apparatus  with  which  I  work  consists,  as  already 
stated,  of  a  glass  tube  about  a  yard  in  length,  and  from 
2%  to  3  inches  internal  diameter.  The  vapour  to  be 
examined  is  introduced  into  this  tube  in  the  manner 
already  described,  and  upon  it  the  condensed  beam  of 
the  electric  lamp  is  permitted  to  act,  until  the  neutral- 
ity or  the  activity  of  the  substance  has  been  declared. 

It  has  hitherto  been  my  aim  to  render  the  chemical 
action  of  light  upon  vapours  visible.  For  this  purpose 
substances  have  been  chosen,  one  at  least  of  whose 
products  of  decomposition  under  light  shall  have  a  boil- 
ing-point so  high,  that  as  soon  as  the  substance  is  formed 
it  shall  be  precipitated.  By  graduating  the  quantity 
of  the  vapour,  this  precipitation  may  be  rendered  of 
any  degree  of  fineness,  forming  particles  distinguishable 
by  the  naked  eye,  or  far  beyond  the  reach  of  our  highest 
microscopic  powers.  I  have  no  reason  to  doubt  that 
particles  may  be  thus  obtained,  whose  diameters  con- 
stitute but  a  small  fraction  ff  the  length  of  a  wave  of 
violet  light. 

In  all  cases  when  the  vapours  of  the  liquids  em- 


ARTIFICIAL    SKY.  HI 

ployed  are  sufficiently  attenuated,  no  matter  what  the 
liquid  may  be,  the  visible  action  commences  with  the 
formation  of  a  blue  cloud.  But  here  I  must  guard  my- 
self against  all  misconception  as  to  the  use  of  this  term. 
The  '  cloud '  here  referred  to  is  totally  invisible  in  or- 
dinary daylight.  To  be  seen,  it  requires  to  be  sur- 
rounded by  darkness,  it  only  being  illuminated  by  a 
powerful  beam  of  light.  This  blue  cloud  differs  in 
many  important  particulars  from  the  finest  ordinary 
clouds,  and  might  justly  have  assigned  to  it  an  inter- 
mediate position  between  such  clouds  and  true  vapour. 
With  this  explanation,  the  term  '  cloud/  or  '  incipient 
cloud,'  or  'actinic  cloud/  as  I  propose  to  employ  it, 
cannot,  I  think,  be  misunderstood. 

I  had  been  endeavouring  to  decompose  carbonic 
acid  gas  by  light.  A  faint  bluish  cloud,  due  it  may  be, 
or  it  may  not  be,  to  the  residue  of  some  vapour  pre- 
viously employed,  was  formed  in  the  experimental  tube. 
On  looking  across  this  cloud  through  a  Nicol's  prism, 
the  line  of  vision  being  horizontal,  it  was  found  that 
,when  the  short  diagonal  of  the  prism  was  vertical,  the 
quantity  of  light  reaching  the  eye  was  greater  than 
when  the  long  diagonal  was  vertical.  When  a  plate  of 
tourmaline  was  held  between  the  eye  and  the  bluish 
cloud,  the  quantity  of  light  reaching  the  eye  when  the 
axis  of  the  prism  was  perpendicular  to  the  axis  of  the 
illuminating  beam,  was  greater  than  when  the  axes  of 
the  crystal  and  of  the  beam  were  parallel  to  each  other. 

This  was  the  result  all  round  the  experimental  tube. 
Causing  the  crystal  of  tourmaline  to  revolve  round  the 
tube,  with  its  axis  perpendicular  to  the  illuminating 
beam,  the  quantity  of  light  that  reached  the  eye  was  in 
all  its  positions  a  maximum.  When  the  crystallographic 
axis  was  parallel  to  the  axis  of  the  beam,  the  quantity 
of  light  transmitted  by  the  crystal  was  a  minimum. 


112  FKAGMEXTS    OF    SCIENCE. 

From  the  illuminated  bluish  cloud,  therefore,  polarised 
light  was  discharged,  the  direction  of  maximum  polari- 
sation being  at  right  angles  to  the  illuminating  beam; 
the  plane  of  vibration  of  the  polarised  light  was  per- 
pendicular to  the  beam.* 

Thin  plates  of  selenite  or  of  quartz,  placed  between 
the  Nicol  and  the  actinic  cloud,  displayed  the  colours  of 
polarised  light,  these  colours  being  most  vivid  when  the 
line  of  vision  was  at  right  angles  to  the  experimental 
tube.  The  plate  of  selenite  usually  employed  was  a 
circle,  thinnest  at  the  centre,  and  augmenting  uniformly 
in  thickness  from  the  centre  outwards.  When  placed 
in  its  proper  position  between  the  Mcol  and  the  cloud, 
it  exhibited  a  system  of  splendidly-coloured  rings. 

The  cloud  here  referred  to  was  the  first  operated 
upon  in  the  manner  described.  It  may,  however,  be 
greatly  improved  upon  by  the  choice  of  proper  sub- 
stances, and  by  the  application,  in  proper  quantities,  of 
the  substances  chosen.  Benzol,  bisulphide  of  carbon, 
nitrite  of  amyl,  nitrite  of  butyl,  iodide  of  allyl,  iodide  of 
isopropyl,  and  many  other  substances  may  be  employed. 
I  will  take  the  nitrite  of  butyl  as  illustrative  of  the 
means  adopted  to  secure  the  best  result,  with  reference 
to  the  present  question. 

And  here  it  may  be  mentioned  that  a  vapour,  which 
when  alone,  or  mixed  with  air  in  the  experimental  tube, 
resists  the  action  of  light,  or  shows  but  a  feeble  result 
of  this  action,  may,  when  placed  in  proximity  with 
another  gas  or  vapour,  exhibit  vigorous,  if  not  violent 
action.  The  case  is  similar  to  that  of  carbonic  acid 
gas,  which,  diffused  in  the  atmosphere,  resists  the  de- 

*  This  is  still  an  undecided  point ;  but  the  probabilities  are  so 
much  in  its  favour,  and  it  is  in  my  opinion  so  much  preferable 
to  have  a  physical  image  on  which  the  mind  can  rest,  that  I  do 
not  hesitate  to  employ  the  phraseology  in  the  text. 


ARTIFICIAL    SKY.  113 

composing  action  of  solar  light,  but  when  placed  in 
contiguity  with  chlorophyl  in  the  leaves  of  plants,  has 
its  molecules  shaken  asunder. 

Dry  air  was  permitted  to  bubble  through  the  liqiiid 
nitrite  of  butyl,  until  the  experimental  tube,  which  had 
been  previously  exhausted,  was  filled  with  the  mixed 
air  and  vapour.  The  visible  action  of  light  upon  the 
mixture  after  fifteen  minutes'  exposure  was  slight. 
The  tube  was  afterwards  filled  with  half  an  atmosphere 
of  the  mixed  air  and  vapour,  and  a  second  half-at- 
mosphere of  air  which  had  been  permitted  to  bubble 
through  fresh  commercial  hydrochloric  acid.  On  send- 
ing the  beam  through  this  mixture,  the  tube,  for  a 
moment,  was  optically  empty.  But  the  pause  amounted 
only  to  a  small  fraction  of  a  second,  a  dense  cloud  being 
immediately  precipitated  upon  the  beam. 

This  cloud  began  blue,  but  the  advance  to  whiteness 
was  so  rapid  as  almost  to  justify  the  application  of  the 
term  instantaneous.  The  dense  cloud,  looked  at  per- 
pendicularly to  its  axis,  showed  scarcely  any  signs  of 
polarisation.  Looked  at  obliquely  the  polarisation  was 
strong. 

The  experimental  tube  being  again  cleansed  and 
exhausted,  the  mixed  air  and  nitrite-of-butyl  vapour 
was  permitted  to  enter  it  until  the  associated  mercury 
column  was  depressed  ^  of  an  inch.  In  other  words, 
the  air  and  vapour,  united,  exercised  a  pressure  not  ex- 
ceeding ^v  of  an  atmosphere.  Air,  passed  through 
a  solution  of  hydrochloric  acid,  was  then  added,  till  the 
mercury  column  was  depressed  three  inches.  The  con- 
densed beam  of  the  electric  light  was  passed  for  some 
time  through  this  mixture  without  revealing  anything 
within  the  tube  competent  to  scatter  the  light.  Soon, 
however,  a  superbly  blue  cloud  was  formed  along  the 
track  of  the  beam,  and  it  continued  blue  sufficiently 


114  FKAGMENTS    OF    SCIENCE. 

long  to  permit  of  its  thorough  examination.  The  light 
discharged  from  the  cloud,  at  right  angles  to  its  own 
length,  was  at  first  perfectly  polarised.  It  could  be 
totally  quenched  by  the  Nicol.  By  degrees  the  cloud 
became  of  whitish  blue,  and  for  a  time  the  selenite 
colours,  obtained  by  looking  at  it  normally,  were  ex- 
ceedingly brilliant.  The  direction  of  maximum  polari- 
sation was  distinctly  at  right  angles  to  the  illuminating 
beam.  This  continued  to  be  the  case  as  long  as  the 
cloud  maintained  a  decided  blue  colour,  and  even  for 
some  time  after  the  blue  had  changed  to  whitish  blue. 
But,  as  the  light  continued  to  act,  the  cloud  became 
coarser  and  whiter,  particularly  at  its  centre,  where  it 
at  length  ceased  to  discharge  polarised  light  in  the  di- 
rection of  the  perpendicular,  while  it  continued  to  do 
so  at  both  ends. 

But  the  cloud  which  had  thus  ceased  to  polarise  the 
light  emitted  normally,  showed  vivid  selenite  colours 
when  looked  at  obliquely,  proving  that  the  direction  of 
maximum  polarisation  changed  with  the  texture  of  the 
cloud.  This  point  shall  receive  further  illustration 
subsequently. 

A  blue,  equally  rich  and  more  durable,  was  obtained 
by  employing  the  nitrite-of-butyl  vapour  in  a  still  more 
attenuated  condition.  The  instance  here  cited  is  rep- 
resentative. In  all  cases,  and  with  all  substances,  the 
cloud  formed  at  the  commencement,  when  the  pre- 
cipitated particles  are  sufficiently  fine,  is  blue,  and.  it  can 
be  made  to  display  a  colour  rivalling  that  of  the  purest 
Italian  sky.  In  all  cases,  moreover,  this  fine  blue  cloud 
polarises  perfectly  the  beam  which  illuminates  it,  the 
direction  of  polarisation  enclosing  an  angle  of  90°  with 
the  axis  of  the  illuminating  beam. 

It  is  exceedingly  interesting  to  observe  both  the 
perfection  and  the  decay  of  this  polarisation.  For  ten 


ARTIFICIAL    SKY.  115 

or  fifteen  minutes  after  its  first  appearance  the  light 
from  a  vividly  illuminated  actinic  cloud,  looked  at  per- 
pendicularly, is  absolutely  quenched  by  a  Nicol's  prism 
with  its  longer  diagonal  vertical.  But  as  the  sky-blue 
is  gradually  rendered  impure  by.  the  growth  of  the 
particles — in  other  words,  as  real  clouds  begin  to  be 
formed — the  polarisation  begins  to  decay,  a  portion  of 
the  light  passing  through  the  prism  in  all  its  positions. 
It  is  worthy  of  note,  that  for  some  time  after  the  cessa- 
tion of  perfect  polarisation,  the  residual  light  which 
passes,  when  the  Nicol  is  in  its  position  of  minimum 
transmission,  is  of  a  gorgeous  blue,  the  whiter  light  of 
the  cloud  being  extinguished.*  When  the  cloud  texture 
has  become  sufficiently  coarse  to  approximate  to  that 
of  ordinary  clouds,  the  rotation  of  the  Mcol  ceases  to 
have  any  sensible  effect  on  the  quantity  of  light  dis- 
charged normally. 

The  perfection  of  the  polarisation,  in  a  direction 
perpendicular  to  the  illuminating  beam,  is  also  illus- 
trated by  the  following  experiment:  A  Nicol's  prism, 
large  enough  to  embrace  the  entire  beam  of  the  electric 
lamp,  was  placed  between  the  lamp  and  the  experi- 
mental tube.  A  few  bubbles  of  air,  carried  through 
the  liquid  nitrite  of  butyl,  were  introduced  into  the 
tube,  and  they  were  followed  by  about  three  inches 
(measured  by  the  mercurial  gauge)  of  air  which  had 
passed  through  aqueous  hydrochloric  acid.  Sending 
the  polarised  beam  through  the  tube,  I  placed  myself 
in  front  of  it,  my  eye  being  on  a  level  with  its  axis,  my 
assistant  occupying  a  similar  position  behind  the  tube. 
The  short  diagonal  of  the  large  Nicol  was  in  the  first 
instance  vertical,  the  plane  of  vibration  of  the  emergent 
beam  being  therefore  also  vertical.  As  the  light  con- 

*  This  shows  that  particles  too  large  to  polarise  the  blue, 
polarise  perfectly  light  of  lower  refrangibility. 


116  FKAGMENTS    OF    SCIENCE. 

tinued  to  act,  a  superb  blue  cloud,  visible  to  both  my 
assistant  and  myself,  was  slowly  formed.  But  this' 
cloud,  so  deep  and  rich  when  looked  at  from  the  posi- 
tions mentioned,  utterly  disappeared  when  looked  at 
vertically  downwards,  or  vertically  upwards.  Keflec- 
tion  from  the  cloud  was  not  possible  in  these  directions. 
When  the  large  JSTicol  was  slowly  turned  round  its  axis, 
the  eye  of  the  observer  being  on  the  level  of  the  beam, 
and  the  line  of  vision  perpendicular  to  it,  entire  extinc- 
tion of  the  light  emitted  horizontally  occurred  when 
the  longer  diagonal  of  the  large  Nicol  was  vertical.  But 
now  a  vivid  blue  cloud  was  seen  when  looked  at  down- 
wards or  upwards.  This  truly  fine  experiment,  which 
I  contemplated  making  on  my  own  account,  was  first 
definitely  suggested  by  a  remark  in  a  letter  addressed 
to  me  by  Professor  Stokes. 

As  regards  the  polarisation  of  sky  light,  the  greatest 
stumbling-block  has  hitherto  been,  that,  in  accordance 
with  the  law  of  Brewster,  which  makes  the  index  of 
refraction  the  tangent  of  the  polarising  angle,  the  re- 
flection which  produces  perfect  polarisation  would  re- 
quire to  be  made  in  air  upon  air;  and  indeed  this  led 
many  of  our  most  eminent  men,  Brewster  himself 
among  the  number,  to  entertain  the  idea  of  aerial 
molecular  reflection.*  I  have,  however,  operated  upon 

*  '  The  cause  of  the  polarisation  is  evidently  a  reflection  of 
the  sun's  light  upon  something.  The  question  is  on  what  I  Were 
the  angle  of  maximum  polarisation  76°,  we  should  look  to  water 
or  ice  as  the  reflecting  body,  however  inconceivable  the  existence 
in  a  cloudless  atmosphere  and  a  hot  summer's  day  of  unevapo- 
rated  molecules  (particles  ?)  of  water.  But  though  we  were  once 
of  this  opinion,  careful  observation  has  satisfied  us  that  90°,  or 
thereabouts,  is  the  correct  angle,  and  that  therefore  whatever  be 
the  body  on  which  the  light  has  been  reflected,  if  polarised  by  a 
single  reflection,  the  polarising  angle  must  be  45°,  and  the  index 
of  refraction,  which  is  the  tangent  of  that  angle,  unity ;  in  other 


ARTIFICIAL    SKY.  117 

substances  of  widely  different  refractive  indices,  and 
therefore  of  very  different  polarising  angles  as  ordi- 
narily defined,  but  the  polarisation  of  the  beam,  by  the 
incipient  cloud,  has  thus  far  proved  itself  to  be  abso- 
lutely independent  of  the  polarising  angle.  The  law 
of  Brewster  does  not  apply  to  matter  in  this  condition, 
and  it  rests  with  the  undulatory  theory  to  explain  why. 
Whenever  the  precipitated  particles  are  sufficiently  fine, 
no  matter  what  the  substance  forming  the  particles 
may  be,  the  direction  of  maximum  polarisation  is  at 
right  angles  to  the  illuminating  beam,  the  polarising 
angle  for  matter  in  this  condition  being  invariably  45°. 
Suppose  our  atmosphere  surrounded  by  an  envelope 
impervious  to  light,  but  with  an  aperture  on  the  sun- 
ward side  through  which  a  parallel  beam  of  solar  light 
could  enter  and  traverse  the  atmosphere.  Surrounded 
by  air  not  directly  illuminated,  the  track  of  such  a 
beam  would  resemble  that  of  the  parallel  beam  of  the 
electric  lamp  through  an  incipient  cloud.  The  sun- 
beam would  be  blue,  and  it  would  discharge  laterally 
light  in  precisely  the  same  condition  as  that  discharged 
by  the  incipient  cloud.  In  fact,  the  azure  revealed 
by  such  a  beam  would  be  to  all  intents  and  purposes 
that  which  I  have  called  a  'blue  cloud/-  Conversely 
our  'blue  cloud'  is,  to  all  intents  and  purposes,  an 
artificial  sky* 

words,  the  reflection  would  require  to  be  made  in  air  upon  air ! ' 
(Sir  John  Herschel, 4  Meteorology,'  par.  233.) 

Any  particles,  if  small  enough,  will  produce  both  the  colour 
and  the  polarisation  of  the  sky.  But  is  the  existence  of  small 
water-particles  on  a  hot  summer's  day  tn  the  higher  regions  of 
our  atmosphere  inconceivable  f  It  is  to  be  remembered  that  the 
oxygen  and  nitrogen  of  the  air  behave  as  a  vacuum  to  radiant 
heat,  the  exceedingly  attenuated  vapour  of  the  higher  atmos- 
phere being  therefore  in  practical  contact  with  the  cold  of  space. 

*  The  opinion  of  Sir  John  Ilerschel,  connecting  the  polarisa- 


118  FKAGMENTS    OF    SCIENCE. 

But,  as  regards  the  polarisation  of  the  sky,  we  know 
that  not  only  is  the  direction  of  maximum  polarisation 
at  right  angles  to  the  track  of  the  solar  beams,  but  that 
at  certain  angular  distances,  probably  variable  ones, 
from  the  sun,  '  neutral  points,'  or  points  of  no  polarisa- 
tion, exist,  on  both  sides  of  which  the  planes  of  at- 
mospheric polarisation  are  at  right  angles  to  each  other. 
I  have  made  various  observations  upon  this  subject 
which  are  reserved  for  the  present;  but,  pending  the 
more  complete  examination  of  the  question,  the  follow- 
ing facts  bearing  upon  it  may  be  submitted. 

The  parallel  beam  employed  in  these  experiments 
tracked  its  way  through  the  laboratory  air,  exactly  as 
sunbeams  are  seen  to  do  in  the  dusty  air  of  London.  I 
have  reason  to  believe  that  a  great  portion  of  the  matter 
thus  floating  in  the  laboratory  air  consists  of  organic 
particles,  which  are  capable  of  imparting  a  perceptibly 
bluish  tint  to  the  air.  These  also  showed,  though  far 
less  vividly,  all  the  effects  of  polarisation  obtained  with 
the  incipient  clouds.  The  light  discharged  laterally 
from  the  track  of  the  illuminating  beam  was  polarised, 
though  not  perfectly,  the  direction  of  maximum  polar- 
isation being  at  right  angles  to  the  beam.  At  all  points 
of  the  beam,-  moreover,  throughout  its  entire  length, 
the  light  emitted  normally  was  in  the  same  state  of 

tion  and  the  blue  colour  of  the  sky,  is  verified  by  the  foregoing 
results.  '  The  more  the  subject  [the  polarisation  of  sky  light]  is 
considered,'  writes  this  eminent  philosopher,  '  the  more  it  will  be 
found  beset  with  difficulties,  and  its  explanation  when  arrived  at 
will  probably  be  found  to  carry  with  it  that  of  the  blue  colour  of 
the  sky  itself,  and  of  the  great  quantity  of  light  it  actually  does 
send  down  to  us.'  '  We  may  observe,  too,'  he  adds,  '  that  it  is 
only  where  the  purity  of  the  sky  is  most  absolute  that  the  polari- 
sation is  developed  in  its  highest  degree,  and  that  where  there  is 
the  slightest  perceptible  tendency  to  cirrus  it  is  materially  im- 
paired.' This  applies  word  for  word  to  our  '  incipient  clouds.' 


ARTIFICIAL    SKY.  119 

polarisation.  Keeping  the  positions  of  the  Nicol  and 
the  selenite  constant,  the  same  colours  were  observed 
throughout  the  entire  beam,  when  the  line  of  vision 
was  perpendicular  to  its  length. 

The  horizontal  column  of  air,  thus  illuminated,  was 
18  feet  long,  and  could  therefore  be  looked  at  very 
obliquely.  I  placed  myself  near  the  end  of  the  beam, 
as  it  issued  from  the  electric  lamp,  and,  looking  through 
the  Nicol  and  selenite  more  and  more  obliquely  at  the 
beam,  observed  the  colours  fading  until  they  disap- 
peared. Augmenting  the  obliquity  the  colours  ap- 
peared once  more,  but  they  were  now  complementary 
to  the  former  ones. 

Hence  this  beam,  like  the  sky,  exhibited  a  neutral 
point,  on  opposite  sides  of  which  the  light  was  polarised 
in  planes  at  right  angles  to  each  other. 

Thinking  that  the  action  observed  in  the  laboratory 
might  be  caused,  in  some  way,  by  the  vaporous  fumes 
diffused  in  its  air,  I  had  the  light  removed  to  a  room  at 
the  top  of  the  Eoyal  Institution.  The  track  of  the 
beam  was  seen  very  finely  in  the  air  of  this  room,  a 
length  of  14  or  15  feet  being  attainable.  This  beam 
exhibited  all  the  effects  observed  with  the  beam -in  the 
laboratory.  Even  the  uncondensed  electric  light  fall- 
ing on  the  floating  matter  showed,  though  faintly,  the 
effects  of  polarisation. 

When  the  air  was  so  sifted  as  to  entirely  remove  the 
visible  floating  matter,  it  no  longer  exerted  any  sensible 
action  upon  the  light,  but  behaved  like  a  vacuum.  The 
light  is  scattered  and  polarised  by  particles,  not  by 
molecules  or  atoms. 

By  operating  upon  the  fumes  of  chloride  of  ammo- 
nium, the  smoke  of  brown  paper,  and  tobacco-smoke,  I 
had  varied  and  confirmed  in  many  ways  those  experi- 
ments on  neutral  points,  when  my  attention  was  drawn 
I 


120  FKAGMENTS    OF    SCIENCE. 

by  Sir  Charles  Wheatstone  to  an  important  observation 
communicated  to  the  Paris  Academy  in  1860  by  Pro- 
fessor Govi,  of  Turin,*  M.  Govi  had  been  led  to  ex- 
amine a  beam  of  light  sent  through  a  room  in  which 
were  successively  diffused  the  smoke  of  incense,  and  to- 
bacco-smoke. His  first  brief  communication  stated  the 
fact  of  polarisation  by  such  smoke;  but  in  his  second 
communication  he  announced  the  discovery  of  a  neu- 
tral point  in  the  beam,  at  the  opposite  sides  of  which 
the  light  was  polarised  in  planes  at  right  angles  to  each 
other. 

But  unlike  my  observations  on  the  laboratory  air, 
and  unlike  the  action  of  the  sky,  the  direction  of  maxi- 
mum polarisation  in  M.  Govi's  experiment  enclosed  a 
very  small  angle  with  the  axis  of  the  illuminating  beam. 
The  question  was  left  in  this  condition,  and  I  am  not 
aware  that  M.  Govi  or  any  other  investigator  has  pur- 
sued it  further. 

I  had  noticed,  as  before  stated,  that  as  the  clouds 
formed  in  the  experimental  tube  became  denser,  the 
polarisation  of  the  light  discharged  at  right  angles  to 
the  beam  became  weaker,  the  direction  of  maximum 
polarisation  becoming  oblique  to  the  beam.  Experi- 
ments on  the  fumes  of  chloride  of  ammonium  gave  me 
also  reason  to  suspect  that  the  position  of  the  neutral 
point  was  not  constant,  but  that  it  varied  with  the 
density  of  the  illuminated  fumes. 

The  examination  of  these  qtiestions  led  to  the  fol- 
lowing new  and  remarkable  results:  The  laboratory  be- 
ing well  filled  with  the  fumes  of  incense,  and  sufficient 
time  being  allowed  for  their  uniform  diffusion,  the 
electric  beam  was  sent  through  the  smoke.  From  the 
track  of  the  beam  polarised  light  was  discharged;  but 
•the  direction  of  maximum  polarisation,  instead  of  being 
*  '  Comptes  Rendus,'  tome  li.  pp.  360  and  669. 


ARTIFICIAL    SKY.  1*^1 

perpendicular,  now  enclosed  an  angle  of  only  12°  or 
1 3°  with  the  axis  of  the  beam. 

A  neutral  point,  with  complementary  effects  at  op- 
posite sides  of  it,  was  also  exhibited  by  the  beam.  The 
angle  enclosed  by  the  axis  of  the  beam,  and  a  line  drawn 
from  the  neutral  point  to  the  observer's  eye,  measured 
in  the  first  instance  66°. 

The  windows  of  the  laboratory  were  now  opened  for 
some  minutes,  a  portion  of  the  incense-smoke  being  per- 
mitted to  escape.  On  again  darkening  the  room  and 
turning  on  the  light,  the  line  of  vision  to  the  neutral 
point  was  found  to  enclose,  with  the  axis  of  the  beam, 
an  angle  of  63°. 

The  windows  were  again  opened  for  a  few  minutes, 
more  of  the  smoke  being  permitted  to  escape.  Measured 
as  before,  the  angle  referred  to  was  found  to  be  54°. 

This  process  was  repeated  three  additional  times; 
the  neutral  point  was  found  to  recede  lower  and  lower 
down  the  beam,  the  angle  between  a  line  drawn  from 
the  eye  to  the  neutral  point  and  the  axis  of  the  beam 
falling  successively  from  54°  to  49°,  43°  and  33°. 

The  distances,  roughly  measured,  of  the  neutral 
point  from  the  lamp,  corresponding  to  the  foregoing 
series  of  observations,  were  these: — 

1st  observation  ....  2  feet    2  inches. 

2nd        "  ....  2    "      6     " 

3rd         "  ....  2   "    10     " 

4th         "  ....  3    "      2     " 

5th         "  ....  3    "      7     " 

6th         "  ....  4    "      6     " 

At  the  end  of  this  series  of  experiments  the  direc- 
tion of  maximum  polarisation  had  again  become  normal 
to  the  beam. 

The  laboratory  was  next  filled  with  the  fumes  of 
gunpowder.  In  five  successive  experiments,  corre- 


122  FRAGMENTS    OF    SCIENCE. 

spending  to  five  different  densities  of  the  gunpowder- 
smoke,  the  angles  enclosed  between  the  line  of  vision  to 
the  neutral  point  and  the  axis  of  the  beam,  were  63°, 
50°,  47°,  42°,  and  38°  respectively. 

After  the  clouds  of  gunpowder  had  cleared  away, 
the  laboratory  was  filled  with  the  fumes  of  common 
resin,  rendered  so  dense  as  to  be  very  irritating  to  the 
lungs.  The  direction  of  maximum  polarisation  enclosed, 
in  this  case,  an  angle  of  12°,  or  thereabouts,  with  the  axis 
of  the  beam.  Looked  at,  as  in  the  former  instances, 
from  a  position  near  the  electric  lamp,  no  neutral  point 
was  observed  throughout  the  entire  extent  of  the  beam. 

When  this  beam  was  looked  at  normally  through  the 
selenite  and  Nicol,  the  ring-system,  though  not  brilliant, 
was  distinct.  Keeping  the  eye  upon  the  plate  of  sele- 
nite, and  tke  line  of  vision  perpendicular,  the  windows 
were  opened,  the  blinds  remaining  undrawn.  The 
resinous  fumes  slowly  diminished,  and  as  they  did  so  the 
ring-system  became  paler.  It  finally  disappeared. 
Continuing  to  look  in  the  same  direction,  the  rings  re- 
vived, but  now  the  colours  were  complementary  to  the 
former  ones.  TJie  neutral  point  had  passed  me  in  its 
motion  down  the  beam,  consequent  upon  the  attenua- 
tion of  the  fumes  of  resin. 

With  the  fumes  of  chloride  of  ammonium  substan- 
tially the  same  results  were  obtained.  Sufficient,  how- 
ever, has  been  here  stated  to  illustrate  the  variability 
of  the  position  of  the  neutral  point.* 

By  a  puff  of  tobacco-smoke,  or  of  condensed  steam, 
blown  into  the  illuminated  beam,  the  brilliancy  of  the 
selenite  colours  may  be  greatly  enhanced.  But  with 
different  clouds  two  different  effects  are  produced.  Let 

*  Brewster  has  proved  the  variability  of  the  position  of  the 
neutral  point  for  sky  light  with  the  sun's  altitude,  a  result  obvi- 
ously connected  with  the  foregoing  experiments. 


ARTIFICIAL    SKY.  ]23 

the  ring-system  observed  in  the  common  air  be  brought 
to  its  maximum  strength,  and  then  let  an  attenuated 
cloud  of  chloride  of  ammonium  be  thrown  into  the 
beam  at  the  point  looked  at;  the  ring  system  flashes 
out  with  augmented  brilliancy,  but  the  character  of  the 
polarisation  remains  unchanged.  This  is  also  the  case 
when  phosphorus,  or  sulphur,  is  burned  underneath  the 
beam,  so  as  to  cause  the  fine  particles  of  phosphorus 
or  of  sulphur  to  rise  into  the  light.  With  the  sulphur- 
fumes  the  brilliancy  of  the  colours  is  exceedingly  inten- 
sified; but  in  none  of  these  cases  is  there  any  change  in 
the  character  of  the  polarisation. 

But  when  a  puff  of  the  fumes  of  hydrochloric  acid, 
hydriodic  acid,  or  nitric  acid  is  thrown  into  the  beam, 
there  is  a  complete  reversal  of  the  selenite  tints.  Each 
of  these  clouds  twists  the  plane  of  polarization  90°, 
causing  the  centre  of  the  ring-system  to  change  from 
black  to  white,  and  the  rings  themselves  to  emit  their 
complementary  colours.* 

Almost  all  liquids  have  motes  in  them  sufficiently 
numerous  to  polarise  sensibly  the  light,  and  very  beau- 
tiful effects  may  be  obtained  by  simple  artificial  devices. 
When,  for  example,  a  cell  of  distilled  water  is  placed 
in  front  of  the  electric  lamp,  and  a  thin  slice  of  the 
beam  is  permitted  to  pass  through  it,  scarcely  any 
polarised  light  is  discharged,  and  scarcely  any  colour 
produced  with  a  plate  of  selenite.  But  if  a  bit  of  soap 
be  agitated  in  the  water  above  the  beam,  the  moment 
the  infinitesimal  particles  reach  the  light  the  liquid 
sends  forth  laterally  almost  perfectly  polarised  light; 

*  Sir  John  Herschel  suggested  to  me  that  this  change  of  the 
polarisation  from  positive  to  negative  may  indicate  a  change 
from  polarisation  by  reflection  to  polarisation  by  refraction. 
This  thought  repeatedly  occurred  to  me  while  looking  at  the 
effects ;  but  it  will  require  much  following  up  before  it  emerges 
into  clearness. 


124  FKAGMENTS    OF    SCIENCE. 

and  if  the  selenite  be  employed,  vivid  colours  flash 
into  existence.  A  still  more  brilliant  result  is  ob- 
tained with  mastic  dissolved  in  a  great  excess  of 
alcohol. 

The  selenite  rings,  in  fact,  constitute  an  extremely 
delicate  test  as  to  the  collective  quantity  of  individually 
invisible  particles  in  a  liquid.  Commencing  with  dis- 
tilled water,  for  example,  a  thick  slice  of  light  is  neces- 
sary to  make  the  polarisation  of  its  suspended  particles 
sensible.  A  much  thinner  slice  suffices  for  common 
water;  while,  with  Briicke's  precipitated  mastic,  a 
slice  too  thin  to  produce  any  sensible  effect  with  most 
other  liquids,  suffices  to  bring  out  vividly  the  selenite 
colours. 

§  3,  THE  SKY  OF  THE  ALPS. 

The  vision  of  an  object  always  implies  a  differential 
action  on  the  retina  of  the  observer.  The  object  is  dis- 
tinguished from  surrounding  space  by  its  excess  or  de- 
fect of  light  in  relation  to  that  space.  By  altering  the 
illumination,  either  of  the  object  itself  or  of  its  en- 
vironment, we  alter  the  appearance  of  the  object. 
Take  the  case  of  clouds  floating  in  the  atmosphere  with 
patches  of  blue  between  them.  Anything  that  changes 
the  illumination  of  either  alters  the  appearance  of  both, 
that  appearance  depending,  as  stated,  upon  differential 
action.  Now  the  light  of  the  sky,  being  polarised, 
may,  as  the  reader  of  the  foregoing  pages  knows,  be  in 
great  part  quenched  by  a  Mcol's  prism,  while  the  light 
of  a  common  cloud,  being  unpolarised,  cannot  be  thus 
extinguished.  Hence  the  possibility  of  very  remarkable 
variations,  not  only  in  the  aspect  of  the  firmament, 
which  is  really  changed,  but  also  in  the  aspect  of  the 
clouds,  which  have  that  firmament  as  a  background. 
It  is  possible,  for  example,  to  choose  clouds  of  such  a 


ARTIFICIAL   SKY.  125 

depth  of  shade  that  when  the  Nicol  quenches  the  light 
behind  them,  they  shall  vanish,  being  undistinguishable 
from  the  residual  dull  tint  which  outlives  the  extinction 
of  the  brilliancy  of  the  sky.  A  cloud  less  deeply 
shaded,  but  still  deep  enough,  when  viewed  with  the 
naked  eye,  to  appear  dark  on  a  bright  ground,  is  sud- 
denly changed  to  a  white  cloud  on  a  dark  ground  by 
the  quenching  of  the  light  behind  it.  When  a  reddish 
cloud  at  sunset  chances  to  float  in  the  region  of  maxi- 
mum polarisation,  the  quenching  of  the  surrounding 
light  causes  it  to  flash  with  a  brighter  crimson.  Last 
Easter  eve  the  Dartmoor  sky,  which  had  just  been 
cleansed  by  a  snowstorm,  wore  a  very  wild  appearance. 
Round  the  horizon  it  was  of  steely  brilliancy,  while 
reddish  cumuli  and  cirri  floated  southwards.  When  the 
sky  was  quenched  behind  them  these  floating  masses 
seemed  like  dull  embers  suddenly  blown  upon;  they 
brightened  like  a  fire. 

In  the  Alps  we  have  the  most  magnificent  examples 
of  crimson  clouds  and  snows,  so  that  the  effects  just 
referred  to  may  be  here  studied  under  the  best  possible 
conditions.  On  August  23,  1869,  the  evening  Alpen- 
glow  was  very  fine,  though  it  did  not  reach  its  maximum 
depth  and  splendour.  The  side  of  the  Weisshorn  seen 
from  the  Bel  Alp,  being  turned  from  the  sun,  was  tinted 
mauve;  but  I  wished  to  observe  one  of  the  rose-coloured 
buttresses  of  the  mountain.  Such  a  one  was  visible 
from  a  point  a  few  hundred  feet  above  the  hotel.  The 
Matterhorn  also,  though  for  the  most  part  in  shade, 
had  a  crimson  projection,  while  a  deep  ruddy  red 
lingered  along  its  western  shoulder.  Four  distinct 
peaks  and  buttresses  of  the  Dom,  in  addition  to  its 
dominant  head — all  covered  with  pure  snow — were  red- 
dened by  the  light  of  sunset.  The  shoulder  of  the 
Alphubel  was  similarly  coloured,  while  the  great  mass 


126  FRAGMENTS    OF    SCIENCE.  . 

of  the  Fletsehorn  was  all  a-glow,  and  so  was  the  snowy 
spine  of  the  Monte  Leone. 

Looking  at  the  Weisshorn  through  the  Mcol,  the 
glow  of  its  protuberance  was  strong  or  weak  according 
to  the  position  of  the  prism.  The  summit  also  under- 
went striking  changes.  In  one  position  of  the  prism  it 
exhibited  a  pale  white  against  a  dark  background;  in 
the  rectangular  position  it  was  a  dark  mauve  against  a 
light  background.  The  red  of  the  Matterhorn  changed 
in  a  similar  manner;  but  the  whole  mountain  also 
passed  through  wonderful  changes  of  definition.  The 
air  at  the  time  was  filled  with  a  silvery  haze,  in  which 
the  Matterhorn  almost  disappeared.  This  could  be 
wholly  quenched  by  the  Nicol,  and  then  the  mountain 
sprang  forth  with  astonishing  solidity  and  detachment 
from  the  surrounding  air.  The  changes  of  the  Dom 
were  still  more  wonderful.  A  vast  amount  of  light 
could  be  removed  from  the  sky  behind  it,  for  it  occu- 
pied the  position  of  maximum  polarisation.  By  a  little 
practice  with  the  Nicol  it  was  easy  to  render  the  extinc- 
tion of  the  light,  or  its  restoration,  almost  instantaneous. 
When  the  sky  was  quenched,  the  four  minor  peaks  and 
buttresses,  and  the  summit  of  the  Dom,  together  with 
the  shoulder  of  the  Alphubel,  glowed  as  if  set  suddenly 
on  fire.  This  was  immediately  dimmed  by  turning  the 
Nicol  through  an  angle  of  90°.  It  was  not  the  stoppage 
of  the  light  of  the  sky  behind  the  mountains  alone 
which  produced  this  startling  effect;  the  air  between 
them  and  me  was  highly  opalescent,  and  the  quench- 
ing of  this  intermediate  glare  augmented  remarkably 
the  distinctness  of  the  mountains. 

On  the  morning  of  August  24  similar  effects  were 
finely  shown.  At  10  A.  M.  all  three  mountains,  the 
Dom,  the  Matterhorn,  and  the  Weisshorn,  were  power- 
fully affected  by  the  Nicol.  But  in  this  instance  also, 


ARTIFICIAL    SKY.  127 

the  line  drawn  to  the  Dom  being  very  nearly  perpen- 
dicular to  the  solar  beams,  the  effects  on  this  mountain 
were  most  striking.  The  grey  summit  of  the  Matter- 
horn,  at  the  same  time,  could  scarcely  be  distinguished 
from  the  opalescent  haze  around  it;  but  when  the  Nicol 
quenched  the  haze,  the  summit  became  instantly  iso- 
lated, and  stood  out  in  bold  definition.  It  is  to  be 
remembered  that  in  the  production  of  these  effects  the 
only  things  changed  are  the  sky  behind,  and  the  lumi- 
nous haze  in  front  of  the  mountains;  that  these  are 
changed  because  the  light  emitted  from  the  sky  and 
from  the  haze  is  plane  polarised  light,  and  that  the 
light  from  the  snows  and  from  the  mountains,  being 
sensibly  unpolarised,  is  not  directly  affected  by  the 
Nicol.  It  will  also  be  understood  that  it  is  not  the 
interposition  of  the  haze  as  an  opaque  body  that  renders 
the  mountains  indistinct,  but  that  it  is  the  light  of 
the  haze  which  dims  and  bewilders  the  eye,  and  thus 
weakens  the  definition  of  objects  seen  through  it. 

These  results  have  a  direct  bearing  upon  what 
artists  call  '  aerial  perspective.'  As  we  look  from  the 
summit  of  Mont  Blanc,  or  from  a  lower  elevation,  at 
the  serried  crowd  of  peaks,  especially  if  the  mountains 
be  darkly  coloured — covered  with  pines,  for  example — 
every  peak  and  ridge  is  separated  from  the  mountains 
behind  it  by  a  thin  blue  haze  which  renders  the  rela- 
tions of  the  mountains  as  to  distance  unmistakable. 
When  this  haze  is  regarded  through  the  Nicol  perpen- 
dicular to  the  sun's  rays,  it  is  in  many  cases  wholly 
quenched,  because  the  light  which  it  emits  in  this  direc- 
tion is  wholly  polarised.  When  this  happens,  aerial 
perspective  is  abolished,  and  mountains  very  differently 
distant  appear  to  rise  in  the  same  vertical  plane.  Close 
to  the  Bel  Alp,  for  instance,  is  the  gorge  of  the  Massa, 
and  beyond  the  gorge  is  a  high  ridge  darkened  by  pines. 


128  FRAGMENTS    OF    SCIENCE. 

This  ridge  may  be  projected  upon  the  dark  slopes  at 
the  opposite  side  of  the  Rhone  valley,  and  between  both 
we  have  the  blue  haze  referred  to,  throwing  the  dis- 
tant mountains  far  away.  But  at  certain  hours  of  the 
day  the  haze  may  be  quenched,  and  then  the  Massa 
ridge  and  the  mountains  beyond  the  Ehone  seem  almost 
equally  distant  from  the  eye.  The  one  appears,  as  it 
were,  a  vertical  continuation  of  the  other.  The  haze 
varies  with  the  temperature  and  humidity  of  the  at- 
mosphere. At  certain  times  and  places  it  is  almost  as 
blue  as  the  sky  itself;  but  to  see  its  colour,  the  atten- 
tion must  be  withdrawn  from  the  mountains  and  from 
the  trees  which  cover  them.  In  point  of  fact,  the  haze 
is  a  piece  of  more  or  less  perfect  sky;  it  is  produced 
in  the  same  manner,  and  is  subject  to  the  same 
laws,  as  the  firmament  itself.  We  live  in  the  sky,  not 
under  it. 

These  points  were  further  elucidated  by  the  deport- 
ment of  the  selenite  plate,  with  which  the  readers  of 
the  foregoing  pages  are  so  well  acquainted.  On  some 
of  the  sunny  days  of  August  the  haze  in  the  valley  of 
the  Ehone,  as  looked  at  from  the  Bel  Alp,  was  very  re- 
markable. Towards  evening  the  sky  above  the  moun- 
tains opposite  to  my  place  of  observation  yielded  a 
series  of  the  most  splendidly-coloured  iris-rings;  but 
on  lowering  the  selenite  until  it  had  the  darkness  of  the 
pines  at  the  opposite  side  of  the  Ehone  valley,  instead 
of  the  darkness  of  space,  as  a  background,  the  colours 
were  not  much  diminished  in  brilliancy.  I  should 
estimate  the  distance  across  the  valley,  as  the  crow 
flies,  to  the  opposite  mountain,  at  nine  miles;  so  that  a 
body  of  air  of  this  thickness  can,  under  favourable  cir- 
cumstances, produce  chromatic  effects  of  polarisation 
almost  as  vivid  as  those  produced  by  the  sky  itself. 

Again:  the  light  of  a  landscape,  as  of  most  other 


ARTIFICIAL    SKY.  129 

things,  consists  of  two  parts;  the  one,  coming  purely 
from  superficial  reflection,  is  always  of  the  same  colour 
as  the  light  which  falls  upon  the  landscape;  the  other 
part  reaches  us  from  a  certain  depth  within  the  objects 
which  compose  the  landscape,  and  it  is  this  portion  of 
the  total  light  which  gives  these  objects  their  distinc- 
tive colours.  The  white  light  of  the  sun  enters  all 
substances  to  a  certain  depth,  and  is  partly  ejected  by 
internal  reflection;  each  distinct  substance  absorbing 
and  reflecting  the  light,  in  accordance  with  the  laws  of 
its  own  molecular  constitution.  Thus  the  solar  light  is 
sifted  by  the  landscape,  which  appears  in  such  colours 
and  variations  of  colour  as,  after  the  sifting  process, 
reach  the  observer's  eye.  Thus  the  bright  green  of 
grass,  or  the  darker  colour  of  the  pine,  never  comes  to 
us  alone,  but  is  always  mingled  with  an  amount  of  light 
derived  from  superficial  reflection.  A  certain  hard 
brilliancy  is  conferred  upon  the  woods  and  meadows  by 
this  superficially-reflected  light.  Under  certain  cir- 
cumstances, it  may  be  quenched  by  a  Nicol's  prism,  and 
we  then  obtain  the  true  colour  of  the  grass  and  foliage. 
Trees  and  meadows,  thus  regarded,  exhibit  a  richness 
and  softness  of  tint  which  they  never  show  as  long  as 
the  superficial  light  is  permitted  to  mingle*  with  the 
true  interior  emission.  The  needles  of  the  pines  show 
this  effect  very  well,  large-leaved  trees  still  better; 
while  a  glimmering  field  of  maize  exhibits  the  most 
extraordinary  variations  when  looked  at  through  the 
rotating  Nicol. 

Thoughts  and  questions  like  those  here  referred  to 
took  me,  in  August  1869,  to  the  top  of  the  Aletsch- 
horn.  The  effects  described  in  the  foregoing  para- 
graphs were  for  the  most  part  reproduced  on  the  summit 
of  the  mountain.  I  scanned  the  whole  of  the  sky  with 
my  Nicol.  Both  alone,  and  in  conjunction  with  the 


130  FKAGMENTS    OF    SCIENCE. 

selenite,  it  pronounced  the  perpendicular  to  the  solar 
beams  to  be  the  direction  of  maximum  polarisation. 
But  at  no  portion  of  the  firmament  was  the  polarisation 
complete.  The  artificial  sky  produced  in  the  experi- 
ments recorded  in  the  preceding  pages  could,  in  this 
respect,  be  rendered  far  more  perfect  than  the  natural 
one;  while  the  gorgeous  '  residual  blue '  which  makes 
its  appearance  when  the  polarisation  of  the  artificial 
sky  ceases  to  be  perfect,  was  strongly  contrasted  with 
the  lack-lustre  hue  which,  in  the  case  of  the  firmament, 
outlived  the  extinction  of  the  brilliancy.  With  certain 
substances,  however,  artificially  treated,  this  dull  residue 
may  also  be  obtained. 

All  along  the  arc  from  the  Matterhorn  to  Mont 
Blanc  the  light  of  the  sky  immediately  above  the  moun- 
tains was  powerfully  acted  upon  by  the  Nicol.  In 
some  cases  the  variations  of  intensity  were  astonishing. 
I  have  already  said  that  a  little  practice  enables  the  ob- 
server to  shift  the  Mcol  from  one  position  to  another 
so  rapidly  as  to  render  the  alternative  extinction  and 
restoration  of  the  light  immediate.  When  this  was 
done  along  the  arc  to  which  I  have  referred,  the  al- 
ternations of  light  and  darkness  resembled  the  play  of 
sheet  lightning  behind  the  mountains.  There  was  an 
element  of  awe  connected  with  the  suddenness  with 
which  the  mighty  masses,  ranged  along  the  line  referred 
to,  changed  their  aspect  and  definition  under  the  opera- 
tion of  the  prism. 


[In  the  last  edition  of  the  '  Fragments  of  Science '  an  essay 
on  '  Dust  and  Disease '  followed  here ;  but  as  almost  all  my 
writings  on  the  '  Germ  Theory '  are  now  collected  in  a  single 
volume  entitled  'Essays  on  the  Floating  Matter  of  the  Air,' 
'  Dust  and  Disease  '  no  longer  appears  in  the  '  Fragments.'  In 
its  place  I  venture  to  introduce  a  short  article  written  early 
last  year  for  an  important  American  magazine.] 


V. 
THE  SKY* 

TNYITED  to  write  for  the  '  Forum '  an  article  that 
-L  would  have  brought  me  face  to  face  with  '  prob- 
lems of  life  and  mind '  for  which  I  was  at  the  moment 
unprepared,  and  unwilling  to  decline  a  request  so  cour- 
teously made,  I  offered,  if  the  editor  cared  to  accept  it, 
to  send  him  a  contribution  on  the  subject  here  pre- 
sented. 

I  mentioned  this  subject,  thinking  that,  in  addition 
to  its  interest  as  a  fragment  of  *  natural  knowledge,'  it 
might  permit  of  a  glance  at  the  workings  of  the  scien- 
tific mind  when  engaged  on  the  deeper  problems  which 
come  before  it.  In  the  house  of  Science  are  many 
mansions,  occupied  by  tenants  of  diverse  kinds.  Some 
of  them  execute  with  painstaking  fidelity  the  useful 
work  of  observation,  recording  from  day  to  day  the 
aspects  of  Nature,  or  the  indications  of  instruments 
devised  *to  reveal  her  ways.  Others  there  are  who  add 
to  this  capacity  for  observation  a  power  over  the  lan- 
guage of  experiment,  by  means  of  which  they  put  ques- 
tions to  Nature,  and  receive  from  her  intelligible  re- 
plies. There  is,  again,  a  third  class  of  minds,  that 
cannot  rest  content  with  observation  and  experiment, 

*  From  '  The  Forum,'  February  1888. 
131 


132  FKAGMENTS    OF    SCIENCE. 

whose  love  of  causal  unity  tempts  them  perpetually 
to  break  through  the  limitations  of  the  senses,  and  to 
seek  beyond  them  the  roots  and  reasons  of  the  phe- 
nomena which  the  observer  and  experimenter  record. 
To  such  spirits — adventurous  and  firm — we  are  indebted 
for  our  deeper  knowledge  of  the  methods  by  which  the 
physical  universe  is  ordered  and  ruled. 

In  his  efforts  to  cross  the  common  bourne  of  the 
known  and  the  unknown,  the  effective  force  of  the  man 
of  science  must  depend,  to  a  great  extent,  upon  his 
acquired  knowledge.  But  knowledge  alone  will  not  do; 
a  stored  memory  will  not  suffice;  inspiration  must  lend 
its  aid.  Scientific  inspiration,  however,  is  usually,  if 
not  always,  the  fruit  of  long  reflection — of  patiently 
'intending  the  mind,'  as  Newton  phrased  it;  and  as 
Copernicus,  Newton,  and  Darwin  practised  it;  until 
outer  darkness  yields  a  glimmer,  which  in  due  time 
opens  out  into  perfect  intellectual  day.  From  some  of 
his  expressions  it  might  be  inferred  that  Newton  scorned 
hypotheses;  but  he  allows  them,  nevertheless,  an  open 
avenue  to  his  own  mind.  He  propounded  the  famous 
corpuscular  theory  of  light,  illustrating  it  and  defend- 
ing it  with  a  skill,  power,  and  fascination  which  sub- 
sequently won  for  it  ardent  supporters  among  the  best 
intellects  of  the  world.  This  theory,  moreover,  was 
weighted  with  a  supplementary  hypothesis,  which  as- 
cribed to  the  luminif erous  molecules  '  fits  of  easy  re- 
flection and  transmission/  in  virtue  of  which  they  were 
sometimes  repelled  from  the  surfaces  of  bodies  and 
sometimes  permitted  to  pass  through.  Newton  may 
have  scorned  the  levity  with  which  hypotheses  are  some- 
times framed;  but  he  lived  in  an  atmosphere  of  theory, 
which  he,  like  all  profound  scientific  thinkers,  found 
to  be  the  very  breath  of  his  intellectual  life. 

The  theorist  takes  his  conceptions  from  the  world 


THE    SKY.  133 

of  fact,  and  refines  and  alters  them  to  suit  his  needs. 
The  sensation  of  sound  was  known  to  be  produced  by 
aerial  waves  impinging  on  the  auditory  nerve.  Air 
being  a  thing  that  could  be  felt,  and  its  vibrations,  by 
suitable  treatment,  made  manifest  to  the  eye,  there  was 
here  a  physical  basis  for  the  '  scientific  imagination  ' 
to  build  upon.  Both  Hooke  and  Huyghens  built  upon 
it  with  effect.  By  the  illustrious  astronomer  last  named 
the  conception  of  waves  was  definitely  transplanted 
from  its  terrestrial  birthplace  to  a  universal  medium 
whose  undulations  could  only  be  intellectually  dis- 
cerned. Huyghens  did  not  establish  the  undulatory 
theory,  but  he  took  the  first  firm  step  towards  estab- 
lishing it.  Laying  this  theory  at  the  root  of  the  phe- 
nomena of  light,  he  went  a  good  way  towards  showing 
that  these  phenomena  are  the  necessary  outgrowth  of 
the  conception. 

By  analysis  atid  synthesis  Newton  proved  the  white 
light  of  the  sun  to  be  a  skein  of  many  colours.  The 
cause  of  colour  was  a  question  which  immediately  oc- 
cupied his  thoughts;  and  here,  as  in  other  cases,  he 
freely  resorted  to  hypothesis.  He  saw,  with  his  mind's 
eye,  his  luminiferous  corpuscles  crossing  the  bodily  eye, 
and  imparting  successive  shocks  to  the  retina  behind. 
To  differences  of  '  bigness '  in  the  light-awakening 
molecules  "Newton  ascribed  the  different  colour-sensa- 
tions. In  the  undulatory  theory  we  are  also  confronted 
with  the  question  of  colour;  and  here  again,  to  inform 
and  guide  us,  we  have  the  analogy  of  sound.  Aerial 
waves  of  different  lengths,  or  periods,  produce  notes  of 
different  pitch;  and  to  differences  of  wave-length  in 
that  mysterious  medium,  the  all-pervading  ether,  dif- 
ferences of  colour  are  ascribed.  Hooke  had  already 
discoursed  of  '  a  very  quick  motion  that  causes  light,  as 
well  as  a  more  robust  that  causes  heat.'  Newton  had 


134  FRAGMENTS    OF    SCIENCE. 

ascribed  the  sensation  of  red  to  the  shock  of  his  grossest, 
and  that  of  violet  to  the  shock  of  his  finest  luminiferous 
projectiles.  Defining  the  one,  and  displacing  the  other 
of  these  notions,  the  wave-theory  affirms  red  to  be  pro- 
duced by  the  largest,  and  violet  by  the  smallest  waves 
of  the  visible  spectrum.  The  theory  of  undulation  had 
to  encounter  that  fierce  struggle  for  existence  which 
all  great  changes  of  doctrine,  scientific  or  otherwise, 
have  had  to  endure.  Mighty  intellects,  following  the 
mightiest  of  them  all,  were  arrayed  against  it.  But  the 
more  it  was  discussed  the  more  it  grew  in  strength  and 
favour,  until  it  finally  supplanted  its  formidable  rival. 
No  competent  scientific  man  at  the  present  day  accepts 
the  theory  of  emission,  or  refuses  to  accept  the  theory 
of  undulation. 

Boyle  and  Hooke  had  been  fruitful  experimenters 
on  those  beautiful  iridescences  known  as  the  '  colours 
of  thin  plates.'  The  rich  hues  of  the  thin-blown  soap- 
bubble,  of  oil  floating  on  water,  and  of  the  thin  layer  of 
oxide  on  molten  lead,  are  familiar  illustrations  of  these 
iris  colours.  Hooke  showed  that  all  transparent  films, 
if  only  thin  enough,  displayed  such  colours;  and  he 
proved  that  the  particular  colour  displayed  depended 
upon  the  thickness  of  the  film.  Passing  from  solid  and 
liquid  films  to  films  of  air,  he  says:  '  Take  two  small 
pieces  of  ground  and  polished  looking-glass  plate,  each 
about  the  bigness  of  a  shilling;  take  these  two  dry,  and 
with  your  forefingers  and  thumbs  press  them  very  hard 
and  close  together,  and  you  shall  find  that  when  they 
approach  each  other  very  near,  there  will  appear  several 
irises  or  coloured  lines.'  Newton,  bent  on  knowing  the 
exact  relation  between  the  thickness  of  the  film  and  the 
colour  it  produced,  varied  Hooke's  experiment.  Tak- 
ing two  pieces  of  glass,  the  one  plane  and  the  other 
very  slightly  curved,  and  pressing  both  together,  he  ob- 


THE    SKY.  135 

tained  a  film  of  air  of  gradually  increasing  thickness 
from  the  place  of  contact  outwards.  As  he  expected, 
he  found  the  place  of  contact  surrounded  by  a  series  of 
coloured  circles,  still  known  all  over  the  world  as  '  New- 
ton's rings.'  The  colours  of  his  first  circle,  which  im- 
mediately surrounded  a  black  central  spot,  Newton 
called  '  colours  of  the  first  order; '  the  colours  of  the 
second  circle,  '  colours  of  the  second  order,'  and  so  on. 
With  unrivalled  penetration  and  apparent  success,  he 
applied  his  theory  of  *  fits '  to  the  explanation  of  the 
'  rings.'  Here,  however,  the  only  immortal  parts  of  his 
labours  are  his  facts  and  measurements;  his  theory  has 
disappeared.  It  was  reserved  for  the  illustrious  Thomas 
Young,  a  man  of  intellectual  calibre  resembling  that  of 
Newton  himself,  to  prove  that  the  rings  were  produced 
by  the  mutual  action — in  technical  phrase,  '  interfer- 
ence ' — of  the  light-waves  reflected  at  the  two  surfaces 
of  the  film  of  air  inclosed  between  the  plane  and  convex 
glasses.  The  colours  of  thin  plates  were  *  residual  col- 
ours ' — survivals  of  the  white  light  after  the  ravages  of 
interference.  Young  soon  translated  the  theory  of 
'  fits '  into  that  of  '  waves; '  the  measurements  pertain- 
ing to  the  former  being  so  accurate  as  to  render  them 
immediately  available  for  the  purposes  of  the  latter. 

It  is  here  that  Newton's  researches  and  opinions 
touch  the  subject  of  this  article.  The  colour  nearest  to 
the  black  spot,  in  the  experiment  above  described,  was 
a  faint  blue — '  blue  of  the  first  order ' — corresponding 
to  the  film  of  air  when  thinnest.  If  a  solid  or  liquid 
film,  of  the  thickness  requisite  to  produce  this  colour, 
were  broken  into  bits  and  scattered  in  the  air,  Newton 
inferred  that  the  tiny  fragments  would  display  the  blue 
colour.  Tantamount  to  this,  he  considered,  was  the  ac- 
tion of  minute  water-particles  in  the  incipient  stage  of 
their  condensation  from  aqueous  vapour.  Such  parti- 
10 


136  FRAGMENTS    OF    SCIENCE. 

cles  suspended  in  our  atmosphere  ought,  he  supposed, 
to  generate  the  serenest  skies.  Newton  does  not  ap- 
pear to  have  bestowed  much  thought  upon  this  subject; 
for  to  produce  the  particular  blue  which  he  regarded 
as  sky-blue,  thin  plates  with  parallel  surfaces  would  be 
required.  The  notion  that  cloud-particles  are  hollow 
spheres,  or  vesicles,  is  prevalent  on  the  Continent,  but  it 
never  made  any  way  among  the  scientific  men  of  Eng- 
land. De  Saussure  thought  that  he  had  actually  seen 
the  cloud-vesicles,  and  Faraday,  as  I  learned  from  him- 
self, believed  that  he  had  once  confirmed  the  observa- 
tion of  the  illustrious  Alpine  traveller.  During  my 
long  acquaintance  with  the  atmosphere  of  the  Alps  I 
have  often  sought  for  these  aqueous  bladders,  but  have 
never  been  able  to  find  them.  Clausius  once  published 
a  profound  essay  on  the  colours  of  the  sky.  The  as- 
sumption of  small  water  drops,  he  proved,  would  lead 
to  optical  consequences  entirely  at  variance  with  facts. 
For  a  time,  therefore,  he  closed  with  the  idea  of  vesicles, 
and  endeavoured  to  deduce  from  them  the  blue  of  the 
firmament  and  the  morning  and  evening  red. 

It  is  not,  however,  necessary  to  invoke  the  blue  of 
the  first  order  to  explain  the  colour  of  the  sky;  nor  is 
it  necessary  to  impose  upon  condensing  vapour  the 
difficult,  if  not  impossible,  task  of  forming  bladders, 
when  it  passes  into  the  liquid  condition.  Let  us  ex- 
amine the  subject.  Eau-de-Cologne  is  prepared  by  dis- 
solving aromatic  gums  or  resins  in  alcohol.  Dropped 
into  water,  the  scented  liquid  immediately  produces  a 
white  cloudiness,  due  to  the  precipitation  of  the  sub- 
stances previously  held  in  solution.  The  solid  parti- 
cles are,  however,  comparatively  gross;  but  by  dimin- 
ishing the  quantity  of  the  dissolved  gum,  the  precipitate 
may  be  made  to  consist  of  extremely  minute  particles. 
Briicke,  for  example,  dissolved  gum-mastic,  in  certain 


THE    SKY.  137 

proportions,  in  alcohol,  and  carefully  dropping  his  solu- 
tion into  a  beaker  of  water,  kept  briskly  stirred,  he 
was  able  to  reduce  the  precipitate  to  an  extremely  fine 
state  of  division.  The  particles  of  mastic  can  by  no 
means  be  imagined  as  forming  bladders.  Still,  against 
a  dark  ground — black  velvet,  for  example — the  water 
that  contains  them  shows  a  distinctly  blue  colour.  The 
bluish  colour  of  many  liquids  is  produced  in  a  similar 
manner.  Thin  milk  is  an  example.  Blue  eyes  are 
also  said  to  be  simply  turbid  media.  The  rocks  over 
which  glaciers  pass  are  finely  ground  and  pulverised  by 
the  ice,  or  the  stony  emery  imbedded  in  it;  and  the 
river  which  issues  from  the  snout  of  every  glacier  is 
laden  with  suspended  matter.  When  such  glacier  water 
is  placed  in  a  tall  glass  jar,  and  the  heavier  particles 
are  permitted  to  subside,  the  liquid  column,  when 
viewed  against  a  dark  background,  has  a  decidedly 
bluish  tinge.  The  exceptional  blueness  of  the  Lake  of 
Geneva,  which  is  fed  with  glacier  water,  may  be  due, 
in  part,  to  particles  small  enough  to  remain  suspended 
long  after  their  larger  and  heavier  companions  have 
sunk  to  the  bottom  of  the  lake. 

We  need  not,  however,  resort  to  water  for  the  pro- 
duction of  the  colour.  We  can  liberate,  in  air,  parti- 
cles of  a  size  capable  of  producing  a  blue  as  deep  and 
pure  as  the  azure  of  the  firmament.  In  fact,  artificial 
skies  may  be  thus  generated,  which  prove  their  brother- 
hood with  the  natural  sky  by  exhibiting  all  its  phe- 
nomena. There  are  certain  chemical  compounds — ag- 
gregates of  molecules — the  constituent  atoms  of  which 
are  readily  shaken  asunder  by  the  impact  of  special 
waves  of  light.  Probably,  if  not  certainly,  the  atoms 
and  the  waves  are  so  related  to  each  other,  as  regards 
vibrating  period,  that  the  wave-motion  can  accumu- 
late until  it  becomes  disruptive.  A  great  number  of 


138  FRAGMENTS    OF    SCIENCE. 

substances  might  be  mentioned  whose  vapours,  when 
mixed  with  air  and  subjected  to  the  action  of  a  solar  or 
an  electric  beam,  are  thus  decomposed,  the  products  of 
decomposition  hanging  as  liquid  or  solid  particles  in  the 
beam  which  generates  them.  And  here  I  must  appeal 
to  the  inner  vision  already  spoken  of.  Eemembering  the 
different  sizes  of  the  waves  of  light,  it  is  not  difficult  to 
see  that  our  minute  particles  are  larger  with  respect  to 
some  waves  than  to  others.  In  the  case  of  water,  for 
example,  a  pebble  will  intercept  and  reflect  a  larger 
fractional  part  of  a  ripple  than  of  a  larger  wave.  We 
have  now  to  imagine  light-undulations  of  different  di- 
mensions, but  all  exceedingly  minute,  passing  through 
air  laden  with  extremely  small  particles.  It  is  plain 
that  such  particles,  though  scattering  portions  of  all 
the  waves,  will  exert  their  most  conspicuous  action  upon 
the  smallest  ones;  and  that  the  colour-sensation  answer- 
ing to  the  smallest  waves — in  other  words,  the  colour 
blue — will  be  predominant  in  the  scattered  light.  This 
harmonises  perfectly  with  what  we  observe  in  the  firma- 
ment. The  sky  is  blue,  but  the  blue  is  not  pure.  On 
looking  at  the  sky  through  a  spectroscope,  we  observe  all 
the  colours  of  the  spectrum;  blue  is  merely  the  pre- 
dominant colour.  By  means  of  our  artificial  skies  we 
can  take,  as  it  were,  the  firmament  in  our  hands  and  ex- 
amine it  at  our  leisure.  Like  the  natural  sky,  the  arti- 
ficial one  shows  all  the  colours  of  the  spectrum,  but  blue 
in  excess.  Mixing  very  small  quantities  of  vapour  with 
air,  and  bringing  the  decomposing  luminous  beam  into 
action,  we  produce  particles  too  small  to  shed  any  sensi- 
ble light,  but  which  may,  and  doubtless  do,  exert  an 
action  on  the  ultra-violet  waves  of  the  spectrum.  We 
can  watch  these  particles,  or  rather  the  space  they  occu- 
py, till  they  grow  to  a  size  able  to  yield  the  firma- 
mental  azure.  As  the  particles  grow  larger  under  the 


THE    SKY..  139 

continued  action  of  the  light,  the  azure  becomes  less 
deep;  while  later  on  a  milkiness,  such  as  we  often  ob- 
serve in  nature,  takes  the  place  of  the  purer  blue. 
Finally  the  particles  become  large  enough  to  reflect  all 
the  light-waves,  and  then  the  suspended  '  actinic  cloud  ' 
diffuses  white  light. 

It  must  occur  to  the  reader  that  even  in  the  absence 
of  definite  clouds  there  are  considerable  variations  in 
the  hue  of  the  firmament.  Everybody  knows,  more- 
over, that  as  the  sky  bends  towards  the  horizon,  the 
purer  blue  is  impaired.  To  measure  the  intensity  of 
the  colour  De  Saussure  invented  a  cyanometer,  and 
Humboldt  has  given  us  a  mathematical  formula  to  ex- 
press the  diminution  of  the  blue,  in  arcs  drawn  east  and 
west  from  the  zenith  downwards.  This  diminution  is 
a  natural  consequence  of  the  predominance  of  coarser 
particles  in  the  lower  regions  of  the  atmosphere.  Were 
the  particles  which  produce  the  purer  celestial  vault  all 
swept  away,  we  should,  unless  helped  by  what  has  been 
called  '  cosmic  dust,'  look  into  the  blackness  of  celestial 
space.  And  were  the  whole  atmosphere  abolished  along 
with  its  suspended  matter,  we  should  have  the  '  black- 
ness '  spangled  with  steady  stars;  for  the  twinkling  of 
the  stars  is  caused  by  our  atmosphere.  Now,  the  higher 
we  ascend,  the  more  do  we  leave  behind  us  the  particles 
which  scatter  the  light;  the  nearer,  in  fact,  do  we  ap- 
proach to  that  vision  of  celestial  space  mentioned  a  mo- 
ment ago.  Viewed,  therefore,  from  the  loftiest  Alpine 
summits,  the  firmamental  blue  is  darker  than  it  is  ever 
observed  to  be  from  the  plains. 

It  is  thus  shown  that  by  the  scattering  action  of 
minute  particles  the  blue  of  the  sky  can  be  produced; 
but  there  is  yet  more  to  be  said  upon  the  subject.  Let 
the  natural  sky  be  looked  at  on  a  fine  day  through  a 
piece  of  transparent  Iceland  spar  cut  into  the  form 


140  FRAGMENTS    OF    SCIENCE. 

known  as  a  Nicol  prism.  It  may  be  well  to  begin  by 
looking  through  the  prism  at  a  snow  slope,  or  a  white 
wall.  Turning  the  prism  round  its  axis,  the  light  com- 
ing from  these  objects  does  not  undergo  any  sensible 
change.  But  when  the  prism  is  directed  towards  the 
sky  the  great  probability  is  that,  on  turning  it,  variations 
in  the  amount  of  light  reaching  the  eye  will  be  observed. 
Testing  various  portions  of  the  sky  with  due  diligence, 
we  at  length  discover  one  particular  direction  where  the 
difference  of  illumination  becomes  a  maximum.  Here 
the  Nicol,  in  one  position,  seems  to  offer  no  impediment 
to  the  passage  of  the  sky  light;  while,  when  turned 
through  an  arc  of  ninety  degrees  from  this  position,  the 
light  is  almost  entirely  quenched.  We  soon  discern  that 
the  particular  line  of  vision  in  which  this  maximum 
difference  is  observed  is  perpendicular  to  the  direction 
of  the  solar  rays.  The  Nicol  acts  thus  upon  sky  light 
because  that  light  is  polarised,  while  the  light  from  the 
white  wall  or  the  white  snow,  being  unpolarised,  is  not 
affected  by  the  rotation  of  the  prism. 

In  the  case  of  our  manufactured  sky  not  only  is  the 
azure  of  the  firmament  reproduced,  but  these  phe- 
nomena of  polarisation  are  observed  even  more  perfectly 
than  in  the  natural  sky.  When  the  air-space  from 
which  our  best  artificial  azure  is  emitted  is  examined 
with  the  Mcol  prism,  the  blue  light  is  found  to  be  com- 
pletely polarised  at  right  angles  to  the  illuminating 
beam.  The  artificial  sky  may,  in  fact,  be  employed  as  a 
second  Nicol,  between  which  and  a  prism  held  in  the 
hand  many  of  the  beautiful  chromatic  phenomena  ob- 
served in  an  ordinary  polariscope  may  be  reproduced. 

Let  us  now  complete  our  thesis  by  following  the 
larger  light-waves,  which  have  been  able  to  pass  among 
the  aerial  particles  with  comparatively  little  fractional 
loss.  Without  going  beyond  inferential  considerations, 


THE    SKY.  141 

we  can  state  what  must  occur.  The  action  of  the  par- 
ticles upon  the  solar  light  increases  with  the  atmospheric 
distances  traversed  by  the  sun's  rays.  The  lower  the 
sun,  therefore,  the  greater  the  action.  The  shorter 
waves  of  the  spectrum  being  more  and  more  withdrawn, 
the  tendency  is  to  give  the  longer  waves  an  enhanced 
predominance  in  the  transmitted  light.  The  tendency, 
in  other  words,  of  this  light,  as  the  rays  traverse  ever- 
increasing  distances,  is  more  and  more  towards  red. 
This,  I  say,  might  be  stated  as  an  inference,  but  it  is 
borne  out  in  the  most  impressive  manner  by  facts. 
When  the  Alpine  sun  is  setting,  or,  better  still,  some 
time  after  he  has  set,  leaving  the  limbs  and  shoulders  of 
the  mountains  in  shadow,  while  their  snowy  crests  are 
bathed  by  the  retreating  light,  the  snow  glows  with 
a  beauty  and  solemnity  hardly  equalled  by  any  other 
natural  phenomenon.  So,  also,  when  first  illumined 
by  the  rays  of  the  unrisen  sun,  the  mountain  heads,  un- 
der favourable  atmospheric  conditions,  shine  like  rubies. 
And  all  this  splendour  is  evoked  by  the  simple  mech- 
anism of  minute  particles,  themselves  without  colour, 
suspended  in  the  air.  Those  who  referred  the  extraor- 
dinary succession  of  atmospheric  glows,  witnessed  some 
years  ago,  to  a  vast  and  violent  discharge  of  volcanic 
ashes,  were  dealing  with  '  a  true  cause/  The  fine  float- 
ing residue  of  such  ashes  would,  undoubtedly,  be  able 
to  produce  the  effects  ascribed  to  it.  Still,  the  mechan- 
ism necessary  to  produce  the  morning  and  the  evening 
red,  though  of  variable  efficiency,  is  always  present  in 
the  atmosphere.  I  have  seen  displays,  equal  in  magnifi- 
cence to  the  finest  of  those  above  referred  to,  when 
there  was  no  special  volcanic  outburst  to  which  they 
could  be  referred.  It  was  the  long-continued  repeti- 
tion of  the  glows  which  rendered  the  volcanic  theory 
highly  probable. 


VI. 

VOYAGE  TO  ALGERIA   TO   OBSERVE  THE 
ECLIPSE. 

1870. 

rr\  HE  opening  of  the  Eclipse  Expedition  was  not  pro- 
-L  pitious.  Portsmouth,  on  Monday,  December  5, 
1870,  was  swathed  by  fog,  which  was  intensified  by 
smoke,  and  traversed  by  a  drizzle  of  fine  rain.  At 
six  P.  M.  I  was  on  board  the  '  Urgent.'  On  Tuesday 
morning  the  weather  was  too  thick  to  permit  of  the 
ship's  being  swung  and  her  compasses  calibrated.  The 
Admiral  of  the  port,  a  man  of  very  noble  presence,  came 
on  board.  Under  his  stimulus  the  energy  which  the 
weather  had  damped  appeared  to  become  more  active, 
and  soon  after  his  departure  we  steamed  down  to  Spit- 
head.  Here  the  fog  had  so  far  lightened  as  to  enable 
the  officers  to  swing  the  ship. 

At  three  p.  M.  on  Tuesday,  December  6,  we  got  away, 
gliding  successively  past  Whitecliff  Bay,  Bembridge, 
Sandown,  Shanklin,  Ventnor,  and  St.  Catherine's 
Lighthouse.  On  Wednesday  morning  we  sighted  the 
Isle  of  Ushant,  on  the  French  side  of  the  Channel. 
The  northern  end  of  the  island  has  been  fretted  by  the 
waves  into  detached  tower-like  masses  of  rock  of  very 
remarkable  appearance.  In  the  Channel  the  sea  was 
green,  and  opposite  Ushant  it  was  a  brighter  green. 
On  Wednesday  evening  we  committed  ourselves  to  the 
Bay  of  Biscay.  The  roll  of  the  Atlantic  was  full,  but 
not  violent.  There  had  been  scarcely  a  gleam  of  sun- 
shine throughout  the  day,  but  the  cloud-forms  were 
142 


VOYAGE    TO    ALGERIA.  143 

fine,  and  their  apparent  solidity  impressive.  On  Thurs- 
day morning  the  green  of  the  sea  was  displaced  by  a 
deep  indigo  blue.  The  whole  of  Thursday  we  steamed 
across  the  bay.  We  had  little  blue  sky,  but  the  clouds 
were  again  grand  and  varied — cirrus,  stratus,  cumulus, 
and  nimbus,  we  had  them  all.  Dusky  hair-like  trails 
were  sometimes  dropped  from  the  distant  clouds  to  the 
sea.  These  were  falling  showers,  and  they  sometimes 
occupied  the  whole  horizon,  while  we  steamed  across 
the  rainless  circle  which  was  thus  surrounded.  Some- 
times we  plunged  into  the  rain,  and  once  or  twice,  by 
slightly  changing  our  course,  avoided  a  heavy  shower. 
From  time  to  time  perfect  rainbows  spanned  the  heavens 
from  side  to  side.  At  times  a  bow  would  appear  in 
fragments,  showing  the  keystone  of  the  arch  midway  in 
air,  and  its  two  buttresses  on  the  horizon.  In  all  cases 
the  light  of  the  bow  could  be  quenched  by  a  Nicol's 
prism,  with  its  long  diagonal  tangent  to  the  arc. 
Sometimes  gleaming  patches  of  the  firmament  were 
seen  amid  the  clouds.  When  viewed  in  the  proper 
direction,  the  gleam  could  be  quenched  by  a  Nicol's 
prism,  a  dark  aperture  being  thus  opened  into  stellar 
space. 

At  sunset  on  Thursday  the  denser  clouds  were 
fiercely  fringed,  while  through  the  lighter  ones  seemed 
to  issue  the  glow  of  a  conflagration.  On  Friday  morn- 
ing we  sighted  Cape  Finisterre — the  extreme  end  of 
the  arc  which  sweeps  from  Ushant  round  the  Bay  of 
Biscay.  Calm  spaces  of  blue,  in  which  floated  quietly 
scraps  of  cumuli,  were  behind  us,  but  in  front  of  us  was 
a  horizon  of  portentous  darkness.  It  continued  thus 
threatening  throughout  the  day.  Towards  evening  the 
wind  strengthened  to  a  gale,  and  at  dinner  it  was  dim- 
cult  to  preserve  the  plates  and  dishes  from  destruction. 
Our  thinned  company  hinted  that  the  rolling  had  other 


144  FRAGMENTS    OF    SCIENCE. 

consequences.  It  was  very  wild  when  we  went  to  bed. 
I  slumbered  and  slept,  but  after  some  time  was  rendered 
anxiously  conscious  that  my  body  had  become  a  kind  of 
projectile,  with  the  ship's  side  for  a  target.  I  gripped 
the  edge  of  my  berth  to  save  myself  from  being  thrown 
out.  Outside,  I  could  hear  somebody  say  that  he  had 
been  thrown  from  his  berth,  and  sent  spinning  to  the 
other  side  of  the  saloon.  The  screw  laboured  violently 
amid  the  lurching;  it  incessantly  quitted  the  water, 
and,  twirling  in  the  air,  rattled  against  its  bearings, 
causing  the  ship  to  shudder  from  stem  to  stern.  At  times 
the  waves  struck  us,  not  with  the  soft  impact  which 
might  be  expected  from  a  liquid,  but  with  the  sudden 
solid  shock  of  battering-rams.  '  No  man  knows  the 
force  of  water/  said  one  of  the  officers,  t  until  he  has 
experienced  a  storm  at  sea/  These  blows  followed  each 
other  at  quicker  intervals,  the  screw  rattling  after  each 
of  them,  until,  finally,  the  delivery  of  a  heavier  stroke 
than  ordinary  seemed  to  reduce  the  saloon  to  chaos. 
Furniture  crashed,  glasses  rang,  and  alarmed  enquiries 
immediately  followed.  Amid  the  noises  I  heard  one 
note  of  forced  laughter;  it  sounded  very  ghastly.  Men 
tramped  through  the  saloon,  and  busy  voices  were  heard 
aft,  as  if  something  there  had  gone  wrong. 

I  rose,  and  not  without  difficulty  got  into  my  clothes. 
In  the  after-cabin,  under  the  superintendence  of  the 
able  and  energetic  navigating  lieutenant,  Mr.  Brown,  a 
group  of  blue-jackets  were  working  at  the  tiller-ropes. 
These  had  become  loose,  and  the  helm  refused  to  answer 
the  wheel.  High  moral  lessons  might  be  gained  on 
shipboard,  by  observing  what  steadfast  adherence  to 
an'  object  can  accomplish,  and  what  large  effects  are 
heaped  up  by  the  addition  of  infinitesimals.  The  tiller- 
rope,  as  the  blue-jackets  strained  in  concert,  seemed 
hardly  to  move;  still  it  did  move  a  little,  until  finally, 


VOYAGE   TO   ALGERIA.  145 

by  timing  the  pull  to  the  lurching  of  the  ship,  the 
mastery  of  the  rudder  was  obtained.  I  had  previously 
gone  on  deck.  Round  the  saloon-door  were  a  few 
members  of  the  eclipse  party,  who  seemed  in  no  mood 
for  scientific  observation.  Nor  did  I;  but  I  wished 
to  see  the  storm.  I  climbed  the  steps  to  the  poop, 
exchanged  a  word  with  Captain  Toynbee,  the  only 
member  of  the  party  to  be  seen  on  the  poop,  and  by 
his  direction  made  towards  a  cleat  not  far  from  the 
wheel.*  Round  it  I  coiled  my  arms.  With  the  excep- 
tion of  the  men  at  the  wheel,  who  stood  as  silent  as 
corpses,  I  was  alone. 

I  had  seen  grandeur  elsewhere,  but  this  was  a  new 
form  of  grandeur  to  me.  The  '  Urgent '  is  long  and 
narrow,  and  during  our  expedition  she  lacked  the 
steadying  influence  of  sufficient  ballast.  She  was  for 
a  time  practically  rudderless,  and  lay  in  the  trough  of 
the  sea.  I  could  see  the  long  ridges,  with  some  hun- 
dreds of  feet  between  their  crests,  rolling  upon  the  ship 
perfectly  parallel  to  her  sides.  As  they  approached, 
they  so  grew  upon  the  eye  as  to  render  the  expression 
' mountains  high'  intelligible.  At  all  events,  there 
was  no  mistaking  their  mechanical  might,  as  they  took 
the  ship  upon  their  shoulders,  and  swung  her  like  a 
pendulum.  The  deck  sloped  sometimes  at  an  angle 
which  I  estimated  at  over  forty-five  degrees;  wanting 
my  previous  Alpine  practice,  I  should  have  felt  less 
confidence  in  my  grip  of  the  cleat.  Here  and  there  the 
long  rollers  were  tossed  by  interference  into  heaps  of 
greater  height.  The  wind  caught  their  crests,  and  scat- 
tered them  over  the  sea,  the  whole  surface  of  which  was 
seething  white.  The  aspect  of  the  clouds  was  a  fit  ac- 
companiment to  the  fury  of  the  ocean.  The  moon  was 

*  The  cleat  is  a  T-shaped  mass  of  metal  employed  for  the 

fastening  of  ropes. 


146  FRAGMENTS    OF    SCIENCE. 

almost  full — at  times  concealed,  at  times  revealed,  as  the 
scud  flew  wildly  over  it.  These  things  appealed  to  the 
eye,  while  the  ear  was  filled  by  the  groaning  of  the  screw 
and  the  whistle  and  boom  of  the  storm. 

Nor  was  the  outward  agitation  the  only  object  of 
interest  to  me.  I  was  at  once  subject  and  object  to 
myself,  and  watched  with  intense  interest  the  workings 
of  my  own  mind.  The  '  Urgent '  is  an  elderly  ship. 
She  had  been  built,  I  was  told,  by  a  contracting  firm 
for  some  foreign  government,  and  had  been  diverted 
from  her  first  purpose  when  converted  into  a  troop-ship. 
She  had  been  for  some  time  out  of  work,  and  I  had 
heard  that  one  of  her  boilers,  at  least,  needed  repair. 
Our  scanty  but  excellent  crew,  moreover,  did  not  belong 
to  the  'Urgent/  but  had  been  gathered  from  other 
ships.  Our  three  lieutenants  were  also  volunteers.  All 
this  passed  swiftly  through  my  mind  as  the  steamer 
shook  under  the  blows  of  the  waves,  and  I  thought  that 
probably  no  one  on  board  could  say  how  much  of  this 
thumping  and  straining  the  '  Urgent '  would  be  able  to 
bear.  This  uncertainty  caused  me  to  look  steadily  at 
the  worst,  and  I  tried  to  strengthen  myself  in  the  face 
of  it. 

But  at  length  the  helm  laid  hold  of  the  water,  and 
the  ship  was  got  gradually  round  to  face  the  waves. 
The  rolling  diminished,  a  certain  amount  of  pitching 
taking  its  place.  Our  speed  had  fallen  from  eleven 
knots  to  two.  I  went  again  to  bed.  After  a  space  of 
calm,  when  we  seemed  crossing  the  vortex  of  a  storm, 
heavy  tossing  recommenced.  I  was  afraid  to  allow 
myself  to  fall  asleep,  as  my  berth  was  high,  and  to  be 
pitched  out  of  it  might  be  attended  with  bruises,  if  not 
with  fractures.  From  Friday  at  noon  to  Saturday  at 
noon  we  accomplished  sixty-six  miles,  or  an  average  of 
less  than  three  miles  an  hour.  I  overheard  the  sailors 


VOYAGE    TO    ALGERIA.  147 

talking  about  this  storm.  The  '  Urgent/  according  to 
those  that  knew  her,  had  never  previously  experienced 
anything  like  it.* 

All  through  Saturday  the  wind,  though  somewhat 
sobered,  blew  dead  against  us.  The  atmospheric  effects 
were  exceedingly  fine.  The  cumuli  resembled  moun- 
tains in  shape,  and  their  peaked  summits  shone  as  white 
as  Alpine  snows.  At  one  place  this  resemblance  was 
greatly  strengthened  by  a  vast  area  of  cloud,  uniformly 
illuminated,  and  lying  like  a  neve  below  the  peaks. 
From  it  fell  a  kind  of  cloud-river  strikingly  like  a 
glacier.  The  horizon  at  sunset  was  remarkable — spaces 
of  brilliant  green  between  clouds  of  fiery  red.  Rain- 
bows had  been  frequent  throughout  the  day,  and  at 
night  a  perfectly  continuous  lunar  bow  spanned  the 
heavens  from  side  to  side.  Its  colours  were  feeble; 
but,  contrasted  with  the  black  ground  against  which  it 
rested,  its  luminousness  was  extraordinary. 

Sunday  morning  found  us  opposite  to  Lisbon,  and 
at  midnight  we  rounded  Cape  St.  Vincent,  where  the 
lurching  seemed  disposed  to  recommence.  Through 
the  kindness  of  Lieutenant  Walton,  a  cot  had  been 
slung  for  me.  It  hung  between  a  tiller-wheel  and  a 
flue,  and  at  one  A.  M.  I  was  roused  by  the  banging  of  the 
cot  against  its  boundaries.  But  the  wind  was  now  be- 
hind us,  and  we  went  along  at  a  speed  of  eleven  knots. 
We  felt  certain  of  reaching  Cadiz  by  three.  But  a  new 
lighthouse  came  in  sight,  which  some  affirmed  to  be 
Cadiz  Lighthouse,  while  the  surrounding  houses  were 
declared  to  be  those  of  Cadiz  itself.  Out  of  deference 
to  these  statements,  the  navigating  lieutenant  changed 
his  course,  and  steered  for  the  place.  A  pilot  came  on 

*  There  is,  it  will  be  seen,  a  fair  agreement  between  these  im- 
pressions and  those  so  vigorously  described  by  a  scientific  corre- 
spondent of  the  '  Times.' 


148  FKAGMENTS    OF    SCIENCE. 

board,  and  he  informed  us  that  we  were  before  the 
mouth  of  the  Guadalquivir,  and  that  the  lighthouse  was 
that  of  Cipiona.  Cadiz  was  still  some  eighteen  miles 
distant. 

We  steered  towards  the  city,  hoping  to  get  into  the 
harbour  before  dark.  But  the  pilot  who  would  have 
guided  us  had  been  snapped  up  by  another  vessel,  and 
we  did  not  get  in.  We  beat  about  during  the  night, 
and  in  the  morning  found  ourselves  about  fifteen  miles 
from  Cadiz.  The  sun  rose  behind  the  city,  and  we 
steered  straight  into  the  light.  The  three-towered 
cathedral  stood  in  the  midst,  round  which  swarmed 
apparently  a  multiude  of  chimney-stacks.  A  nearer 
approach  showed  the  chimneys  to  be  small  turrets.  A 
pilot  was  taken  on  board;  for  there  is  a  dangerous  shoal 
in  the  harbour.  The  appearance  of  the  town  as  the 
sun  shone  upon  its  white  and  lofty  walls  was  singularly 
beautiful.  We  cast  anchor;  some  officials  arrived  and 
demanded  a  clean  bill  of  health.  We  had  none.  They 
would  have  nothing  to  do  with  us;  so  the  yellow  quar- 
antine flag  was  hoisted,  and  we  waited  for  permission 
to  land  the  Cadiz  party.  After  some  hours'  delay  the 
English  consul  and  vice-consul  came  on  board,  and  with 
them  a  Spanish  officer  ablaze  with  gold  lace  and  decora- 
tions. Under  slight  pressure  the  requisite  permission 
had  been  granted.  We  landed  our  party,  and  in  the 
afternoon  weighed  anchor.  Thanks  to  the  kindness  of 
our  excellent  paymaster,  I  w"as  here  transferred  to  a 
more  roomy  berth. 

Cadiz  soon  sank  beneath  the  sea,  and  we  sighted  in 
succession  Cape  Trafalgar,  Tarifa,  and  the  revolving 
light  of  Ceuta.  The  water  was  very  calm,  and  the 
moon  rose  in  a  quiet  heaven.  She  swung  with  her  con- 
vex surface  downwards,  the  common  boundary  between 
light  and  shadow  being  almost  horizontal.  A  pillar  of 


VOYAGE    TO   ALGERIA.  149 

reflected  light  shimmered  up  to  us  from  the  slightly 
rippled  sea.  I  had  previously  noticed  the  phosphores- 
cence of  the  water,  but  to  night  it  was  stronger  than 
usual,  especially  among  the  foam  at  the  bows.  A  bucket 
let  down  into  the  sea  brought  up  a  number  of  the  little 
sparkling  organisms  which  caused  the  phosphorescence. 
I  caught  some  of  them  in  my  hand.  And  here  an  ap- 
pearance was  observed  which  was  new  to  most  of  us, 
and  strikingly  beautiful  to  all.  Standing  at  the  bow 
and  looking  forwards,  at  a  distance  of  forty  or  fifty  yards 
from  the  ship,  a  number  of  luminous  streamers  were 
seen  rushing  towards  us.  On  nearing  the  vessel  they 
rapidly  turned,  like  a  comet  round  its  perihelion,  placed 
themselves  side  by  side,  and,  in  parallel  trails  of  light, 
kept  up  with  the  ship.  One  of  them  placed  itself  right 
in  front  of  the  bow  as  a  pioneer.  These  comets  of  the 
sea  were  joined  at  intervals  by  others.  Sometimes  as 
many  as  six  at  a  time  would  rush  at  us,  bend  with  ex- 
traordinary rapidity  round  a  sharp  curve,  and  after- 
wards keep  us  company.  I  leaned  over  the  bow,  and 
scanned  the  streamers  closely.  The  frontal  portion  of 
each  of  them  revealed  the  outline  of  a  porpoise.  The 
rush  of  the  creatures  through  the  water  had  started  the 
phosphorescence,  every  spark  of  which  was  converted  by 
the  motion  of  the  retina  into  a  line  of  light.  Each  por- 
poise was  thus  wrapped  in  a  luminous  sheath.  The 
phosphorescence  did  not  cease  at  the  creature's  tail,  but 
was  carried  many  porpoise-lengths  behind  it. 

To  our  right  we  had  the  African  hills,  illuminated 
by  the  moon.  Gibraltar  Rock  at  length  became  visible, 
but  the  town  remained  long  hidden  by  a  belt  of  haze, 
through  which  at  length  the  brighter  lamps  struggled. 
It  was  like  the  gradual  resolution  of  a  nebula  into 
stars.  As  the  intervening  depth  became  gradually  less, 
the  mist  vanished  more  and  more,  and  finally  all  the 


150  FKAGMENTS    OF    SCIENCE. 

lamps  shone  through  it.  They  formed  a  bright  foil  to 
the  sombre  mass  of  rock  above  them.  The  sea  was  so 
calm  and  the  scene  so  lovely  that  Mr.  Huggins  and  my- 
self stayed  on  deck  till  near  midnight,  when  the  ship  was 
moored.  During  our  walking  to  and  fro  a  striking  en- 
largement of  the  disk  of  Jupiter  was  observed,  when- 
ever the  heated  air  of  the  funnel  came  between  us  and 
the  planet.  On  passing  away  from  the  heated  air,  the 
flat  dim  disk  would  immediately  shrink  to  a  luminous 
point.  The  effect  was  one  of  visual  persistence.  The 
retinal  image  of  the  planet  was  set  quivering  in  all 
azimuths  by  the  streams  of  heated  air,  describing  in 
quick  succession  minute  lines  of  light,  which  summed 
themselves  to  a  disk  of  sensible  area. 

At  six  o'clock  next  morning,  the  gun  at  the  Signal 
Station  on  the  summit  of  the  rock,  boomed.  At  eight 
the  band  on  board  the  '  Trafalgar '  training-ship,  which 
was  in  the  harbour,  struck  up  the  national  anthem;  and 
immediately  afterwards  a  crowd  of  mite-like  cadets 
swarmed  up  the  rigging.  After  the  removal  of  the 
apparatus  belonging  to  the  Gibraltar  party  we  went  on 
shore.  Winter  was  in  England  when  we  left,  but  here 
we  had  the  warmth  of  summer.  The  vegetation  was 
luxuriant — palm-trees,  cactuses,  and  aloes,  all  ablaze 
with  scarlet  flowers.  A  visit  to  the  Governor  was  pro- 
posed, as  an  act  of  necessary  courtesy,  and  I  accom- 
panied Admiral  Ommaney  and  Mr.  Huggins  to  'the 
Convent/  or  Government  House.  We  sent  in  our  cards, 
waited  for  a  time,  and  were  then  conducted  by  an  or- 
derly to  his  Excellency.  He  is  a  fine  old  man,  over 
six  feet  high,  and  of  frank  military  bearing.  He  re- 
ceived us  and  conversed  with  us  in  a  very  genial  man- 
ner. He  took  us  to  see  his  garden,  his  palms,  his 
shaded  promenades,  and  his  orange-trees  loaded  with 
fruit,  in  all  of  which  he  took  manifest  delight.  Evi- 


VOYAGE    TO    ALGERIA.  151 

dently  '  the  hero  of  Kars '  had  fallen  upon  quarters 
after  his  own  heart.  He  appeared  full  of  good  nature, 
and  engaged  us  on  the  spot  to  dine  with  him  that  day. 

We  sought  the  town-major  for  a  pass  to  visit  the 
lines.  While  awaiting  his  arrival  I  purchased  a  stock 
of  white  glass  bottles,  with  a  view  to  experiments  on 
the  colour  of  the  sea.  Mr.  Huggins  and  myself,  who 
wished  to  see  the  rock,  were  taken  by  Captain  Salmond 
to  the  library,  where  a  model  of  Gibraltar  is  kept,  and 
where  we  had  a  useful  preliminary  lesson.  At  the 
library  we  met  Colonel  Maberly,  a  courteous  and  kindly 
man,  who  gave  us  good  advice  regarding  our  excursion. 
He  sent  an  orderly  with  us  to  the  entrance  of  the  lines. 
The  orderly  handed  us  over  to  an  intelligent  Irishman, 
who  was  directed  to  show  us  everything  that  we  de- 
sired to  see,  and  to  hide  nothing  from  us.  We  took 
the  '  upper  line/  traversed  the  galleries  hewn  through 
the  limestone;  looked  through  the  embrasures,  which 
opened  like  doors  in  the  precipice,  towards  the  hills  of 
Spain;  reached  St.  George's  hall,  and  went  still  higher, 
emerging  on  the  summit  of  one  of  the  noblest  cliffs  I 
have  ever  seen. 

Beyond  were  the  Spanish  lines,  marked  by  a  line  of 
white  sentry-boxes;  nearer  were  the  English  lines,  less 
conspicuously  indicated;  and  between  both  was  the 
neutral  ground.  Behind  the  Spanish  lines  rose  the 
conical  hill  called  the  Queen  of  Spain's  Chair.  The 
general  aspect  of  the  mainland  from  the  rock  is  bold 
and  rugged.  Doubling  back  from  the  galleries,  we 
struck  upwards  towards  the  crest,  reached  the  Signal 
Station,  where  we  indulged  in  '  shandy-gaff '  and  bread 
and  cheese.  Thence  to  O'Hara's  Tower,  the  highest 
point  of  the  rock.  It  was  built  by  a  former  Governor, 
who,  forgetful  of  the  laws  of  terrestrial  curvature, 
thought  he  might  look  from  the  tower  into  the  port  of 
11 


152  FRAGMENTS    OF    SCIENCE. 

Cadiz.  The  tower  is  riven,  and  it  may  be  climbed  along 
tbe  edges  of  the  crack.  We  got  to  the  top  of  it;  thence 
descended  the  curious  Mediterranean  Stair — a  zigzag, 
mostly  of  steps  down  a  steeply  falling  slope,  amid 
palmetto  brush,  aloes,  and  prickly  pear. 

Passing  over  the  Windmill  Hill,  we  were  joined  at 
the  '  Governor's  Cottage  '  by  a  car,  and  drove  afterwards 
to  the  lighthouse  at  Europa  Point.  The  tower  was 
built,  I  believe,  by  Queen  Adelaide,  and  it  contains  a 
fine  dioptric  apparatus  of  the  first  order,  constructed  by 
Messrs.  Chance,  of  Birmingham.  At  the  appointed 
hour  we  were  at  the  Convent.  During  dinner  the  same 
genial  traits  which  appeared  in  the  morning  were  still 
more  conspicuous.  The  freshness  of  the  Governor's 
nature  showed  itself  best  when  he  spoke  of  his  old  an- 
tagonist in  arms,  Mouravieff.  Chivalry  in  war  is  con- 
sistent with  its  stern  prosecution.  These  two  men 
were  chivalrous,  and  after  striking  the  last  blow  became 
friends  forever.  Our  kind  and  courteous  reception  at 
Gibraltar  is  a  thing  to  be  remembered  with  pleasure. 

On  December  15  we  committed  ourselves  to  the 
Mediterranean.  The  views  of  Gibraltar  with  which  we 
are  most  acquainted  represent  it  as  a  huge  ridge;  but 
its  aspect,  end  on,  both  from  the  Spanish  lines  and  from 
the  other  side,  is  truly  noble.  There  is  a  sloping  bank 
of  sand  at  the  back  of  the  rock,  which  I  was  disposed 
to  regard  simply  as  the  debris  of  the  limestone.  I 
wished  to  let  myself  down  upon  it,  but  had  not  the  time. 
My  friend  Mr.  Busk,  however,  assures  me  that  it  is 
silica,  and  that  the  same  sand  constitutes  the  adjacent 
neutral  ground.  There  are  theories  afloat  as  to  its  hav- 
ing been  blown  from  Sahara.  The  Mediterranean 
throughout  this  first  day,  and  indeed  throughout  the 
entire  voyage  to  Oran,  was  of  a  less  deep  blue  than  the 
Atlantic.  Possibly  the  quantity  of  organisms  may  have 


VOYAGE    TO    ALGERIA.  153 

modified  the  colour.  At  night  the  phosphorescence 
was  startling,  breaking  suddenly  out  along  the  crests 
of  the  waves  formed  by  the  port  and  starboard  bows. 
Its  strength  was  not  uniform.  Having  flashed  bril- 
liantly for  a  time,  it  would  in  part  subside,  and  after- 
wards regain  its  vigour.  Several  large  phosphorescent 
masses  of  weird  appearance  also  floated  past. 

On  the  morning  of  the  16th  we  sighted  the  fort  and 
lighthouse  of  Marsa  el  Kibir,  and  beyond  them  the 
white  walls  of  Oran  lying  in  the  bight  of  a  bay,  shel- 
tered by  dominant  hills.  The  sun  was  shining  bright- 
ly; during  our  whole  voyage  we  had  not  had  so  fine  a 
day.  The  wisdom  which  had  led  us  to  choose  Oran  as 
our  place  of  observation  seemed  demonstrated.  A 
rather  excitable  pilot  came  on  board,  and  he  guided  us 
in  behind  the  Mole,  which  had  suffered  much  damage 
the  previous  year  from  an  unexplained  outburst  of  waves 
from  the  Mediterranean.  Both  port  and  bow  anchors 
were  cast  in  deep  water.  With  three  huge  hawsers  the 
ship's  stern  was  made  fast  to  three  gun-pillars  fixed  in 
the  Mole;  and  here  for  a  time  the  'Urgent*  rested 
from  her  labours. 

M.  Janssen,  who  had  rendered  his  name  celebrated 
by  his  observations  of  the  eclipse  in  India  in  1868,  when 
he  showed  the  solar  flames  to  be  eruptions  of  incan- 
descent hydrogen,  was  already  encamped  in  the  open 
country  about  eight  miles  from  Oran.  On  December  2 
he  had  quitted  Paris  in  a  balloon,  with  a  strong  young 
sailor  as  his  assistant,  had  descended  near  the  mouth  of 
the  Loire,  seen  M.  Gambetta,  and  received  from  him 
encouragement  and  aid.  On  the  day  of  our  arrival  his 
encampment  was  visited  by  Mr.  Huggins,  and  the  kind 
and  courteous  Engineer  of  the  Port  drove  me  subse- 
quently, in  his  own  phaeton,  to  the  place.  It  bore  the 
best  repute  as  regards  freedom  from  haze  and  fog,  and 


154  FEAGMENTS    OF    SCIENCE. 

commanded  an  open  outlook;  but  it  was  inconvenient 
for  us  on  account  of  its  distance  from  the  ship.  The 
place  next  in  repute  was  the  railway  station,  between 
two  and  three  miles  distant  from  the  Mole.  It  was 
inspected,  but,  being  enclosed,  was  abandoned  for  an 
eminence  in  an  adjacent  garden,  the  property  of  Mr. 
Hinshelwood,  a  Scotchman  who  had  settled  some  years 
previously  as  an  Esparto  merchant  in  Oran.*  He,  in 
the  most  liberal  manner,  placed  his  ground  at  the  dis- 
position of  the  party.  Here  the  tents  were  pitched,  on 
the  Saturday,  by  Captain  Salmond  and  his  intelligent 
corps  of  sappers,  the  instruments  being  erected  on  the 
Monday  under  cover  of  the  tents. 

Close  to  the  railway  station  runs  a  new  loopholed 
wall  of  defence,  through  which  the  highway  passes  into 
the  open  country.  Standing  on  the  highway,  and 
looking  southwards,  about  twenty  yards  to  the  right  is 
a  small  bastionet,  intended  to  carry  a  gun  or  two.  Its 
roof  I  thought  would  form  an  admirable  basis  for  my 
telescope,  while  the  view  of  the  surrounding  country 
was  unimpeded  in  all  directions.  The  authorities  kind- 
ly allowed  me  the  use  of  this  bastionet.  Two  men, 
one  a  blue-jacket  named  Elliot,  and  the  other  a  marine 
named  Hill,  were  placed  at  my  disposal  by  Lieutenant 
Walton;  and,  thus  aided,  on  Monday  morning  I  mount- 
ed my  telescope.  The  instrument  was  new  to  me,  and 
some  hours  of  discipline  were  spent  in  mastering  all  the 
details  of  its  manipulation. 

Mr.  Huggins  joined  me,  and  we  visited  together  the 
Arab  quarter  of  Oran.  The  flat-roofed  houses  appeared 
very  clean  and  white.  The  street  was  filled  with  loiter- 
ers, and  the  thresholds  were  occupied  by  picturesque 
groups.  Some  of  the  men  were  very  fine.  We  saw 

*  Esparto  is  a  kind  of  grass  now  much  used  in  the  manufac- 
ture of  paper. 


VOYAGE   TO   ALGERIA.  155 

many  straight,  manly  fellows  who  must  have  been  six 
feet  four  in  height.  They  passed  us  with  perfect  in- 
difference, evincing  no  anger,  suspicion,  or  curiosity, 
hardly  caring  in  fact  to  glance  at  us  as  we  passed.  .  In 
one  instance  only  during  my  stay  at  Oran  was  I  spoken 
to  by  an  Arab.  He  was  a  tall,  good-humoured  fellow, 
who  came  smiling  up  to  me,  and  muttered  something 
about  '  les  Anglais/  The  mixed  population  of  Oran  is 
picturesque  in  the  highest  degree:  the  Jews,  rich  and 
poor,  varying  in  their  costumes  as  their  wealth  varies; 
the  Arabs  more  picturesque  still,  and  of  all  shades  of 
complexion — the  negroes,  the  Spaniards,  the  French, 
all  grouped  together,  each  race  preserving  its  own  indi- 
viduality, formed  a  picture  intensely  interesting  to  me. 
On  Tuesday,  the  20th,  I  was  early  at  the  bastionet. 
The  night  had  been  very  squally.  The  sergeant  of  the 
sappers  had  taken  charge  of  our  key,  and  on  Tuesday 
morning  Elliot  went  for  it.  He  brought  back  the  in- 
telligence that  the  tents  had  been  blown  down,  and  the 
instruments  overturned.  Among  these  was  a  large  and 
valuable  equatorial  from  the  Eoyal  Observatory,  Green- 
wich. It  seemed  hardly  possible  that  this  instrument, 
with  its  wheels  and  verniers  and  delicate  adjustments, 
could  have  escaped  uninjured  from  such  a  fall.  This, 
however,  was  the  case;  and  during  the  day  all  the  over- 
turned instruments  were  restored  to  their  places,  and 
found  to  be  in  practical  working  order.  This  and  the 
following  day  were  devoted  to  incessant  schooling.  I 
had  come  out  as  a  general  stargazer,  and  not  with  the 
intention  of  devoting  myself  to  the  observation  of  any 
particular  phenomenon.  I  wished  to  see  the  whole — 
the  first  contact,  the  advance  of  the  moon,  the  suc- 
cessive swallowing  up  of  the  solar  spots,  the  breaking 
of  the  last  line  of  crescent  by  the  lunar  mountains  into 
Bailey's  beads,  the  advance  of  the  shadow  through  the 


156  FRAGMENTS    OF    SCIENCE. 

air,  the  appearance  of  the  corona  and  prominences  at 
the  moment  of  totality,  the  radiant  streamers  of  the 
corona,  the  internal  structure  of  the  flames,  a  glance 
through  a  polariscope,  a  sweep  round  the  landscape 
with  the  naked  eye,  the  reappearance  of  the  solar  limb 
through  Bailey's  beads,  and,  finally,  the  retreat  of  the 
lunar  shadow  through  the  air. 

I  was  provided  with  a  telescope  of  admirable  defini- 
tion, mounted,  adjusted,  packed,  and  most  liberally 
placed  at  my  disposal  by  Mr.  Warren  De  La  Eue.  The 
telescope  grasped  the  whole  of  the  sun,  and  a  consider- 
able portion  of  the  space  surrounding  it.  But  it  would 
not  take  in  the  extreme  limits  of  the  corona.  For  this 
I  had  lashed  on  to  the  large  telescope  a  light  but  pow- 
erful instrument,  constructed  by  Eoss,  and  lent  to  me 
by  Mr.  Huggins.  I  was  also  furnished  with  an  excellent 
binocular  by  Mr.  Dallmeyer.  In  fact,  no  man  could 
have  been  more  efficiently  supported.  It  required  a 
strict  parcelling  out  of  the  interval  of  totality  to  em- 
brace in  it  the  entire  series  of  observations.  These, 
while  the  sun  remained  visible,  were  to  be  made  with 
an  unsilvered  diagonal  eye-piece,  which  reflected  but  a 
small  fraction  of  the  sun's  light,  this  fraction  being 
still  further  toned  down  by  a  dark  glass.  At  the  mo- 
ment of  totality  the  dark  glass  was  to  be  removed,  and 
a  silver  reflector  pushed  in,  so  as  to  get  the  maximum 
of  light  from  the  corona  and  prominences.  The  time 
of  totality  was  distributed  as  follows: 

1.  Observe  approach  of  shadow  through  the  air:  totality. 

2.  Telescope 30  seconds. 

3.  Finder 30  seconds. 

4.  Double  image  prism       .        .        .        .15  seconds. 

5.  Naked  eye      .        ....        .        .10  seconds. 

6.  Finder  or  binocular       .        .        .        .20  seconds. 

7.  Telescope 20  seconds. 

8.  Observe  retreat  of  shadow. 


VOYAGE    TO    ALGERIA.  157 

In  our  rehearsals  Elliot  stood  beside  me,  watch  in 
hand,  and  furnished  with  a  lantern.  He  called  out  at 
the  end  of  each  interval,  while  I  moved  from  telscope 
to  finder,  from  finder  to  polariscope,  from  polariscope 
to  naked  eye,  from  naked  eye  back  to  finder,  from 
finder  to  telescope,  abandoning  the  instrument  finally 
to  observe  the  retreating  shadow.  All  this  we  went 
over  twenty  times,  while  looking  at  the  actual  sun,  and 
keeping  him  in  the  middle  of  the  field.  It  was  my 
object  to  render  the  repetition  of  the  lesson  so  mechan- 
ical as  to  leave  no  room  for  flurry,  forgetfulness,  or  ex- 
citement. Volition  was  not  to  be  called  upon,  nor  judg- 
ment exercised,  but  a  well-beaten  path  of  routine  was 
to  be  followed.  Had  the  opportunity  occurred,  I 
think  the  programme  would  have  been  strictly  carried 
out. 

But  the  opportunity  did  not  occur.  For  several 
days  the  weather  had  been  ill-natured.  We  had  wind 
so  strong  as  to  render  the  hawsers  at  the  stern  of  the 
'  Urgent '  as  rigid  as  iron,  and  to  destroy  the  navigating 
lieutenant's  sleep.  We  had  clouds,  a  thunder-storm, 
and  some  rain.  Still  the  hope  was  held  out  that  the  at- 
mosphere would  cleanse  itself,  and  if  it  did  we  were 
promised  air  of  extraordinary  limpidity.  Early  on  the 
22nd  we  were  all  at  our  posts.  Spaces  of  blue  in  the 
early  morning  gave  us  some  encouragement,  but  all  de- 
pended on  the  relation  of  these  spaces  to  the  surround- 
ing clouds.  Which  of  them  were  to  grow  as  the  day  ad- 
vanced? The  wind  was  high,  and  to  secure  the  steadi- 
ness of  my  instrument  I  was  forced  to  retreat  behind  a 
projection  of  the  bastionet,  place  stones  upon  its  stand, 
and,  further,  to  avail  myself  of  the  shelter  of  a  sail.  My 
practised  men  fastened  the  sail  at  the  top,  and  loaded 
it  with  boulders  at  the  bottom.  It  was  tried  severely, 
but  it  stood  firm. 


158  FKAGMENTS    OF    SCIENCE. 

The  clouds  and  blue  spaces  fought  for  a  time  with 
varying  success.  The  sun  was  hidden  and  revealed  at 
intervals,  hope  oscillating  in  synchronism  with  the 
changes  of  the  sky.  At  the  moment  of  first  contact  a 
dense  cloud  intervened;  but  a  minute  or  two  afterwards 
the  cloud  had  passed,  and  the  encroachment  of  the 
black  body  of  the  moon  was  evident  upon  the  solar 
disk.  The  moon  marched  onward,  and  I  saw  it  at 
frequent  intervals;  a  large  group  of  spots  were  ap- 
proached and  swallowed  up.  Subsequently  I  caught 
sight  of  the  lunar  limb  as  it  cut  through  the  middle  of 
a  large  spot.  The  spot  was  not  to  be  distinguished  from 
the  moon,  but  rose  like  a  mountain  above  it.  The 
clouds,  when  thin,  could  be  seen  as  grey  scud  drifting 
across  the  black  surface  of  the  moon;  but  they  thick- 
ened more  and  more,  and  made  the  intervals  of  clear- 
ness scantier.  During  these  moments  I  watched  with 
an  interest  bordering  upon  fascination  the  march  of  the 
silver  sickle  of  the  sun  across  the  field  of  the  telescope. 
It  was  so  sharp  and  so  beautiful.  No  trace  of  the  lunar 
limb  could  be  observed  beyond  the  sun's  boundary. 
Here,  indeed,  it  could  only  be  relieved  by  the  corona, 
which  was  utterly  cut  off  by  the  dark  glass.  The  black- 
ness of  the  moon  beyond  the  sun  was,  in  fact,  con- 
founded with  the  blackness  of  space. 

Beside  me  was  Elliot  with  the  watch  and  lantern, 
while  Lieutenant  Archer,  of  the  Eoyal  Engineers,  had 
the  kindness  to  take  charge  of  my  note-book.  I  men- 
tioned, and  he  wrote  rapidly  down,  such  things  as 
seemed  worthy  of  remembrance.  Thus  my  hands  and 
mind  were  entirely  free;  but  it  was  all  to  no  purpose. 
A  patch  of  sunlight  fell  and  rested  upon  the  landscape 
some  miles  away.  It  was  the  only  illuminated  spot 
within  view.  But  to  the  north-west  there  was  still  a 
space  of  blue  which  might  reach  us  in  time.  Within 


VOYAGE   TO   ALGERIA.  159 

seveii  minutes  of  totality  another  space  towards  the 
zenith  became  very  dark.  The  atmosphere  was,  as  it 
were,  on  the  brink  of  a  precipice,  being  charged  with 
humidity,  which  required  but  a  slight  chill  to  bring  it 
down  in  clouds.  This  was  furnished  by  the  withdrawal 
of  the  solar  beams:  the  clouds  did  come  down,  cover- 
ing up  the  space  of  blue  on  which  our  hopes  had  so 
long  rested.  I  abandoned  the  telescope  and  walked  to 
and  fro  in  despair.  As  the  moment  of  totality  ap- 
proached, the  descent  towards  darkness  was  as  obvious 
as  a  falling  stone.  I  looked  towards  a  distant  ridge, 
where  the  darkness  would  first  appear.  At  the  moment 
a  fan  of  beams,  issuing  from  the  hidden  sun,  was  spread 
out  over  the  southern  heavens.  These  beams  are  bars 
of  alternate  light  and  shade,  produced  in  illuminated 
haze  by  the  shadows  of  floating  cloudlets  of  varying 
density.  The  beams  are  practically  parallel,  but  by  an 
effect  of  perspective  they  appear  divergent,  having  the 
sun,  in  fact,  for  their  point  of  convergence.  The  dark- 
ness took  possession  of  the  ridge  referred  to,  lowered 
upon  M.  Janssen's  observatory,  passed  over  the  southern 
heavens,  blotting  out  the  beams  as  if  a  sponge  had 
been  drawn  across  them.  It  then  took  successive  pos- 
session of  three  spaces  of  blue  sky  in  the  south-eastern 
atmosphere.  I  again  looked  towards  the  ridge.  A 
glimmer  as  of  day-dawn  was  behind  it,  and  immediately 
afterwards  the  fan  of  beams,  which  had  been  for  more 
than  two  minutes  absent,  revived.  The  eclipse  of  1870 
had  ended,  and,  as  far  as  the  corona  and  flames  were 
concerned,  we  had  been  defeated. 

Even  in  the  heart  of  the  eclipse  the  darkness  was  by 
no  means  perfect.  Small  print  could  be  read.  In  fact, 
the  clouds  which  rendered  the  day  a  dark  one,  by  scat- 
tering light  into  the  shadow,  rendered  the  darkness  less 
intense  than  it  would  have  been  had  the  atmosphere 


160  FEAGMENTS    OF    SCIENCE. 

been  without  cloud.  In  the  more  open  spaces  I  sought 
for  stars,  but  could  find  none.  There  was  a  lull  in  the 
wind  before  and  after  totality,  but  during  the  totality 
the  wind  was  strong.  I  waited  for  some  time  on  the 
bastionet,  hoping  to  get  a  glimpse  of  the  moon  on  the 
opposite  border  of  the  sun,  but  in  vain.  The  clouds 
continued,  and  some  rain  fell.  The  day  brightened 
somewhat  afterwards,  and,  having  packed  all  up,  in  the 
sober  twilight  Mr.  Crookes  and  myself  climbed  the 
heights  above  the  fort  of  Vera  Cruz.  From  this  emi- 
nence we  had  a  very  noble  view  over  the  Mediterranean 
and  the  flanking  African  hills.  The  sunset  was  remark- 
able, and  the  whole  outlook  exceedingly  fine. 

The  able  and  well-instructed  medical  officer  of  the 
'  Urgent,'  Mr.  Goodman,  observed  the  following  tem- 
peratures during  the  progress  of  the  eclipse: 

Hour  Deg.  Hour  Deg. 

11.45  .  .  56  12.43  .  .  51 

11.55  .  55  1.5  .  .  52 

12.10  .  .  54  1.27  .  .  53 

12.37  .  .  53  1.44  .  .  56 

12.39  .  .  52  2.10  .  .  57 

The  minimum  temperature  occurred  some  minutes 
after  totality,  when  a  slight  rain  fell. 

The  wind  was  so  strong  on  the  23rd  that  Captain 
Henderson  would  not  venture  out.  Guided  by  Mr. 
Goodman,  I  visited  a  cave  in  a  remarkable  stratum 
of  shell-breccia,  and,  thanks  to  my  guide,  secured  speci- 
mens. Mr.  Busk  informs  me  that  a  precisely  similar 
breccia  is  found  at  Gibraltar,  at  approximately  the  same 
level.  During  the  afternoon,  Admiral  Ommaney  and 
myself  drove  to  the  fort  of  Marsa  el  Kibir.  The  forti- 
fication is  of  ancient  origin,  the  Moorish  arches  being 
still  there  in  decay,  but  the  fort  is  now  very  strong. 
About  four  or  five  hundred  fine-looking  dragoons  were 


VOYAGE    TO    ALGERIA.  161 

looking  after  their  horses,  waiting  for  a  lull  to  enable 
them  to  embark  for  France.  One  of  their  officers  was 
wandering  in  a  very  solitary  fashion  over  the  fort.  We 
liad  some  conversation  with  him.  He  had  been  at  Se- 
dan, had  been  taken  prisoner,  but  had  effected  his  es- 
cape. He  shook  his  head  when  we  spoke  of  the  ter- 
mination of  the  war,  and  predicted  its  long  continuance. 
There  was  bitterness  in  his  tone  as  he  spoke  of  the 
charges  of  treason  so  lightly  levelled  against  French 
commanders.  The  green  waves  raved  round  the  pro- 
montory on  which  the  fort  stands,  smiting  the  rocks, 
breaking  into  foam,  and  jumping,  after  impact,  to  a 
height  of  a  hundred  feet  and  more  into  the  air.  As  we 
returned  our  vehicle  broke  down  through  the  loss  of  a 
wheel.  The  Admiral  went  on  board,  while  I  remained 
long  watching  the  agitated  sea.  The  little  horses  of 
Oran  well  merit  a  passing  word.  Their  speed  and  en- 
durance, both  of  which  are  heavily  drawn  upon  by  their 
drivers,  are  extraordinary. 

The  wind  sinking,  we  lifted  anchor  on  the  24th. 
For  some  hours  we  went  pleasantly  along;  but  during 
the  afternoon  the  storm  revived,  and  it  blew  heavily 
against  us  all  the  night.  When  we  came  opposite  the 
Bay  of  Almeria,  on  the  25th,  the  captain  turned  the 
ship,  and  steered  into  the  bay,  where,  under  the  shadow 
of  the  Sierra  Nevada,  we  passed  Christmas  night  in 
peace.  Next  morning  '  a  rose  of  dawn  '  rested  on  the 
snows  of  the  adjacent  mountains,  while  a  purple  haze 
was  spread  over  the  lower  hills.  I  had  no  notion  that 
Spain  possessed  so  fine  a  range  of  mountains  as  the 
Sierra  Nevada.  The  height  is  considerable,  but  the 
form  also  is  such  as  to  get  the  maximum  of  grandeur 
out  of  the  height.  We  weighed  anchor  at  eight  A.  M., 
passing  for  a  time  through  shoal  water,  the  bottom 
having  been  evidently  stirred  up.  The  adjacent  land 


162  FRAGMENTS    OF    SCIENCE. 

seemed  eroded  in  a  remarkable  manner.  It  has  its 
floods,  which  excavate  these  valleys  and  ravines,  and 
leave  those  singular  ridges  behind.  Towards  evening  I 
climbed  the  mainmast,  and,  standing  on  the  cross-trees, 
saw  the  sun  set  amid  a  blaze  of  fiery  clouds.  The  wind 
was  strong  and  bitterly  cold,  and  I  was  glad  to  slide 
back  to  the  deck  along  a  rope,  which  stretched  from 
the  mast-head  to  the  ship's  side.  That  night  we  cast 
anchor  beside  the  Mole  of  Gibraltar. 

On  the  morning  of  the  27th,  in  company  with  two 
friends,  I  drove  to  the  Spanish  lines,  with  the  view  of 
seeing  the  rock  from  that  side.  It  is  an  exceedingly 
noble  mass.  The  Peninsular  and  Oriental  mail-boat 
had  been  signalled  and  had  come.  Heavy  duties  called 
me  homeward,  and  by  transferring  myself  from  the 
*  Urgent '  to  the  mail-steamer  I  should  gain  three  days. 
I  hired  a  boat,  rowed  to  the  steamer,  learned  that  she 
was  to  start  at  one,  and  returned  with  all  speed  to  the 
'Urgent/  Making  known  to  Captain  Henderson  my 
wish  to  get  away,  he  expressed  doubts  as  to  the  pos- 
sibility of  reaching  the  mail-steamer  in  time.  With  his 
accustomed  kindness,  he  however  placed  a  boat  at  my 
disposal.  Four  hardy  fellows  and  one  of  the  ship's  offi- 
cers jumped  into  it;  my  luggage,  hastily  thrown  to- 
gether, was  tumbled  in,  and  we  were  immediately  on 
our  way.  We  had  nearly  four  miles  to  row  in  about 
twenty  minutes;  but  we  hoped  the  mail-boat  might  not 
be  punctual.  For  a  time  we  watched  her  anxiously; 
there  was  no  motion;  we  came  nearer,  but  the  flags  were 
not  yet  hauled  in.  The  men  put  forth  all  their 
strength,  animated  by  the  exhortations  of  the  officer  at 
the  helm.  The  roughness  of  the  sea  rendered  their  ef- 
forts to  some  extent  nugatory:  still  we  were  rapidly  ap- 
proaching the  steamer.  At  length  she  moved,  punctual 
almost  to  the  minute,  at  first  slowly,  but  soon  with 


VOYAGE    TO    ALGERIA.  163 

quickened  pace.  We  turned  to  the  left,  so  as  to  cut 
across  her  bows.  Five  minutes' pull  would  have  brought 
us  up  to  her.  The  officer  waved  his  cap  and  I  my  hat. 
'  If  they  could  only  see  us,  they  might  back  to  us  in  a 
moment/  But  they  did  not  see  us,  or  if  they  did,  they 
paid  us  no  attention.  I  returned  to  the  '  Urgent,'  dis- 
comfited, but  grateful  to  the  fine  fellows  who  had 
wrought  so  hard  to  carry  out  my  wishes. 

Glad  of  the  quiet,  in  the  sober  afternoon  I  took  a 
walk  towards  Europa  Point.  The  sky  darkened  and 
heavy  squalls  passed  at  intervals.  Private  theatricals 
were  at  the  Convent,  and  the  kind  and  courteous  Gov- 
ernor had  sent  cards  to  the  eclipse  party.  I  failed  in 
my  duty  in  not  going.  St.  Michael's  Cave  is  said  to 
rival,  if  it  does  not  outrival,  the  Mammoth  Cave  of 
Kentucky.  On  the  28th  Mr.  Crookes,  Mr.  Carpenter, 
and  myself,  guided  by  a  military  policeman  who  under- 
stood his  work,  explored  the  cavern.  The  mouth  is 
about  1,100  feet  above  the  sea.  We  zigzagged  up  to  it, 
and  first  were  led  into  an  aperture  in  the  rock,  at  some 
height  above  the  true  entrance  of  the  cave.  In  this  up- 
per cavern  we  saw  some  tall  and  beautiful  stalactite 
pillars. 

The  water  drips  from  the  roof  charged  with  bicar- 
bonate of  lime.  Exposed  to  the  air,  the  carbonic  acid 
partially  escapes,  and  the  simple  carbonate  of  lime, 
which  is  hardly  at  all  soluble  in  water,  deposits  itself  as 
a  solid,  forming  stalactites  and  stalagmites.  Even  the 
exposure  of  chalk  or  limestone  water  to  the  open  air 
partially  softens  it.  A  specimen  of  the  Redbourne 
water  exposed  by  Professors  Graham,  Miller,  and  Hof- 
mann,  in  a  shallow  basin,  fell  from  eighteen  degrees  to 
nine  degrees  of  hardness.  The  softening  process  of 
Clark  is  virtually  a  hastening  of  the  natural  process. 
Here,  however,  instead  of  being  permitted  to  evaporate. 


164  FRAGMENTS    OF    SCIENCE. 

half  the  carbonic  acid  is  appropriated  by  lime,  the  half 
thus  taken  up,  as  well  as  the  remaining  half,  being 
precipitated.  The  solid  precipitate  is  permitted  to  sink, 
and  the  clear  supernatant  liquid  is  limpid  soft  water. 

"We  returned  to  the  real  mouth  of  St.  Michael's 
Cave,  which  is  entered  by  a  wicket.  The  floor  was 
somewhat  muddy,  and  the  roof  and  walls  were  wet.  We 
soon  found  ourselves  in  the  midst  of  a  natural  temple, 
where  tall  columns  sprang  complete  from  floor  to  roof, 
while  incipient  columns  were  growing  to  meet  each 
other,  upwards  and  downwards.  The  water  which 
trickles  from  the  stalactite,  after  having  in  part  yielded 
up  its  carbonate  of  lime,  falls  upon  the  floor  vertically 
underneath,  and  there  builds  the  stalagmite.  Conse- 
quently, the  pillars  grow  from  above  and  below  simul- 
taneously, along  the  same  vertical.  It  is  easy  to  dis- 
tinguish the  stalagmitic  from  the  stalactitic  portion  of 
the  pillars.  The  former  is  always  divided  into  short 
segments  by  protuberant  rings,  as  if  deposited  period- 
ically, while  the  latter  presents  a  uniform  surface.  In 
some  cases  the  points  of  inverted  cones  of  stalactite 
rested  on  the  centres  of  pillars  of  stalagmite.  The 
process  of  solidification  and  the  consequent  architecture 
were  alike  beautiful. 

We  followed  our  guide  through  various  branches 
and  arms  of  the  cave,  climbed  and  descended  steps, 
halted  at  the  edges  of  dark  shafts  and  apertures,  and 
squeezed  ourselves  through  narrow  passages.  From  time 
to  time  we  halted,  while  Mr.  Crookes  illuminated  with 
ignited  magnesium  wire,  the  roof,  columns,  dependent 
spears,  and  graceful  drapery  of  the  stalactites.  Once, 
coming  to  a  magnificent  cluster  of  icicle-like  spears,  we 
helped  ourselves  to  specimens.  There  was  some  diffi- 
culty in  detaching  the  more  delicate  ones,  their  fragility 
was  so  great.  A  consciousness  of  vandalism,  which 


VOYAGE   TO   ALGERIA.  165 

smote  me  at  the  time,  haunts  me  still;  for,  though  our 
requisitions  were  moderate,  this  beauty  ought  not  to  be 
at  all  invaded.  Pendent  from  the  roof,  in  their  natural 
habitat,  nothing  can  exceed  their  delicate  beauty;  they 
live,  as  it  were,  surrounded  by  organic  connections.  In 
London  they  are  curious,  but  not  beautiful.  Of  gath- 
ered shells  Emerson  writes: 

I  wiped  away  the  weeds  and  foam, 
And  brought  my  sea-born  treasures  home : 
But  the  poor,  unsightly,  noisome  things 
Had  left  their  beauty  on  the  shore, 
With  the  sun,  and  the  sand,  and  the  wild  uproar. 

The  promontory  of  Gibraltar  is  so  burrowed  with 
caverns  that  it  has  been  called  the  Hill  of  Caves.  They 
are  apparently  related  to  the  geologic  disturbances 
which  the  rock  has  undergone.  The  earliest  of  these  is 
the  tilting  of  the  once  horizontal  strata.  Suppose  a 
force  of  torsion  to  act  upon  the  promontory  at  its  south- 
ern extremity  near  Europa  Point,  and  suppose  the 
rock  to  be  of  a  partially  yielding  character;  such  a 
force  would  twist  the  strata  into  screw-surfaces,  the 
greatest  amount  of  twisting  being  endured  near  the 
point  of  application  of  the  force.  Such  a  twisting  the 
rock  appears  to  have  suffered;  but  instead  of  the  twist 
fading  gradually  and  uniformly  off,  in  passing  from 
south  to  north,  the  want  of  uniformity  in  the  material 
has  produced  lines  of  dislocation  where  there  are  abrupt 
changes  in  the  amount  of  twist.  Thus,  at  the  northern 
end  of  the  rock  the  dip  to  the  west  is  nineteen  degrees; 
in  the  Middle  Hill,  it  is  thirty-eight  degrees;  in  the 
centre  of  the  South  Hill,  or  Sugar  Loaf,  it  is  fifty-seven 
degrees.  At  the  southern  extremity  of  the  Sugar  Loaf 
the  strata  are  vertical,  while  farther  to  the  south  they 
actually  turn  over  and  dip  to  the  east. 

The  rock  is  thus  divided  into  three  sections,  sepa- 


166  FEAGMENTS    OF    SCIENCE. 

rated  from  each  other  by  places  of  dislocation,  where 
the  strata  are  much  wrenched  and  broken.  These  are 
called  the  Northern  and  Southern  Quebrada,  from  the 
Spanish  '  Tierra  Quebrada/  or  broken  ground.  It  is  at 
these  places  that  the  inland  caves  of  Gibraltar  are 
almost  exclusively  found.  Based  on  the  observations  of 
Dr.  Falconer  and  himself,  an  excellent  and  most  in- 
teresting account  of  these  caves,  and  of  the  human 
remains  and  works  of  art  which  they  contain,  was  com- 
municated by  Mr.  Busk  to  the  meeting  of  the  Congress 
of  Prehistoric  Archeology  at  Norwich,  and  afterwards 
printed  in  the  '  Transactions  '  of  the  Congress.*  Long 
subsequent  to  the  operation  of  the  twisting  force  just 
referred  to,  the  promontory  underwent  various  changes 
of  level.  There  are  sea-terraces  and  layers  of  shell- 
breccia  along  its  flanks,  and  numerous  caves  which, 
unlike  the  inland  ones,  are  the  product  of  marine  ero- 
sion. The  Ape's  Hill,  on  the  African  side  of  the  strait, 
Mr.  Busk  informs  me  has  undergone  similar  disturb- 
ances, f 

In  the  harbour  of  Gibraltar,  on  the  morning  of  our 
departure,  I  resumed  a  series  of  observations  on  the 
colour  of  the  sea.  On  the  way  out  a  number  of  speci- 
mens had  been  collected,  with  a  view  to  subsequent 
examination.  But  the  bottles  were  claret  bottles,  of 
doubtful  purity.  At  Gibraltar,  therefore,  I  purchased 
fifteen  white  glass  bottles,  with  ground  glass  stoppers, 
and  at  Cadiz,  thanks  to  the  friendly  guidance  of  Mr. 
Cameron,  I  secured  a  dozen  more.  These  seven-and- 

*  In  this  essay  Mr.  Busk  refers  to  the  previous  labours  of  Mr. 
Smith,  of  Jordan  Hill,  to  whom  we  owe  most  of  our  knowledge 
of  the  geology  of  the  rock. 

•f-  No  one  can  rise  from  the  perusal  of  Mr.  Busk's  paper  with- 
out a  feeling  of  admiration  for  the  principal  discoverer  and  inde- 
fatigable explorer  of  the  Gibraltar  caves,  the  late  Captain  Fred- 
erick Brome. 


VOYAGE    TO    ALGERIA.  167 

twenty  bottles  were  filled  with  water,  taken  at  differ- 
ent places  between  Oran  and  Spithead. 

And  here  let  me  express  my  warmest  acknowledg- 
ments to  Captain  Henderson,  the  commander  of 
H.  M.  S.  '  Urgent/  who  aided  me  in  my  observations 
in  every  possible  way.  Indeed,  my  thanks  are  due  to 
all  the  officers  for  their  unfailing  courtesy  and  help. 
The  captain  placed  at  my  disposal  his  own  coxswain, 
an  intelligent  fellow  named  Thorogood,  who  skilfully 
attached  a  cord  to  each  bottle,  weighted  it  with  lead, 
cast  it  into  the  sea,  and,  after  three  successive  rinsings, 
filled  it  under  my  own  eyes.  The  contact  of  jugs, 
buckets,  or  other  vessels  was  thus  avoided;  and  even 
the  necessity  of  pouring  out  the  water,  afterwards, 
through  the  dirty  London  air. 

The  mode  of  examination  applied  to  these  bottles 
has  been  already  described.*  The  liquid  is  illuminated 
by  a  powerfully  condensed  beam,  its  condition  being 
revealed  through  the  light  scattered  by  its  suspended 
particles.  '  Care  is  taken  to  defend  the  eye  from  the 
access  of  all  other  light,  and,  thus  defended,  it  becomes 
an  organ  of  inconceivable  delicacy/  Were  water  of 
uniform  density  perfectly  free  from  suspended  matter, 
it  would,  in  my  opinion,  scatter  no  light  at  all.  The 
track  of  a  luminous  beam  could  not  be  seen  in  such 
water.  But  '  an  amount  of  impurity  so  infinitesimal 
as  to  be  scarcely  expressible  in  numbers,  and  the  in- 
dividual particles  of  which  are  so  small  as  wholly  to 
elude  the  microscope,  may,  when  examined  by  the 
method  alluded  to,  produce  not  only  sensible,  but  strik- 
ing, effects  upon  the  eye.' 

The  results  of  the  examination  of  nineteen  bottles 
filled  at  various  places  between  Gibraltar  and  Spit- 
head  are  here  tabulated: 

*  « Floating  Matter  of  the  Air,'  Art « Dust  and  Disease.' 
12 


168 


FKAGMENTS    OF    SCIENCE. 


locality. 

Colour  of  Sea. 

Appearance  in  Luminous  Beam. 

Gibraltar  Harbour  
Two  miles  from  Gibraltar  . 
Off  Cabreta  Point  
Off  Cabreta  Point  
Off  Tarifa 

Green  
Clearer  green. 
Bright  green. 
Black-indigo  . 
Undecided  .  .  . 
Cobalt-blue  .  . 
Yellow-green. 
Yellow-green. 
Yellow-green. 
Bright  green. 

Deep  indigo.. 
Strong  green. 
Indigo  
Undecided  .  .  . 
Black-indigo  . 
Indigo  
Dark  green.  .  . 
Yellow-green. 
Green  

Thick  with  fine  particles. 
Thick  with  very  fine  particles. 
Still  thick,  but  less  so. 
Much  less  thick,  very  pure. 
Thicker  than  No.  4. 
Much  purer  than  No.  5. 
Very  thick. 
Exceedingly  thick. 
Thick,  but  less  so. 
Much  less  thick. 

Very  little  matter,  very  pure. 
Thick,  with  fine  matter. 
Very  little  matter,  pure. 
Less  pure. 
Very  little  matter,  very  pure. 
Very  fine  matter.  Iridescent. 
A  good  deal  of  matter. 
Exceedingly  thick. 
Exceedingly  thick. 

Twelve  miles  from  Cadiz.  . 
Cadiz  Harbour  
Fourteen  miles  from  Cadiz. 
Fourteen  miles  from  Cadiz. 
Betwc-en   Capes   St.   Mary 
and  Vincent  

Beyond  the  Burlings  
Off  Cape  Finisterre  
Bay  of  Biscay  
Bay  of  Biscay  
Off  Ushant  
Off  St.  Catherine's  
Spithead  

Here  we  have  three  specimens  of  water,  described 
as  green,  a  clearer  green,  and  bright  green,  taken  in 
Gibraltar  Harbour,  at  a  point  two  miles  from  the 
harbour,  and  off  Cabreta  Point.  The  home  examina- 
tion showed  the  first  to  be  thick  with  suspended  mat- 
ter, the  second  less  thick,  and  the  third  still  less  thick. 
Thus  the  green  brightened  as  the  suspended  matter 
diminished  in  amount. 

Previous  to  the  fourth  observation  our  excellent 
navigating  lieutenant,  Mr.  Brown,  steered  along  the 
coast,  thus  avoiding  the  adverse  current  which  sets  in, 
through  the  Strait,  from  the  Atlantic  to  the  Mediter- 
ranean. He  was  at  length  forced  to  cross  the  boundary 
of  the  Atlantic  current,  which  was  defined  with  ex- 
traordinary sharpness.  On  the  one  side  of  it  the  water 
was  a  vivid  green,  on  the  other  a  deep  blue.  Standing 
at  the  bow  of  the  ship,  a  bottle  could  be  filled  with 
blue  water,  while  at  the  same  moment  a  bottle  cast 
from  the  stern  could  be  filled  with  green  water.  Two 
bottles  were  secured,  one  on  each  side  of  this  remark- 
able boundary.  In  the  distance  the  Atlantic  had  the 
hue  called  ultra-marine;  but  looked  fairly  down  upon, 


VOYAGE    TO    ALGEKIA.  169 

it  was  of  almost  inky  blackness — black  qualified  by  a 
trace  of  indigo. 

What  change  does  the  home  examination  here 
reveal?  In  passing  to  indigo,  the  water  becomes  sud- 
denly augmented  in  purity,  the  suspended  matter  be- 
coming suddenly  less.  Off  Tarifa,  the  deep  indigo 
disappears,  and  the  sea  is  undecided  in  colour.  Ac- 
companying this  change,  we  have  a  rise  in  the  quantity 
of  suspended  matter.  Beyond  Tarifa,  we  change  to 
cobalt-blue,  the  suspended  matter  falling  at  the  same 
time  in  quantity.  This  water  is  distinctly  purer  than 
the  green.  We  approach  Cadiz,  and  at  twelve  miles 
from  the  city  get  into  yellow-green  water;  this  the 
London  examination  shows  to  be  thick  with  suspended 
matter.  The  same  is  true  of  Cadiz  harbour,  and  also 
of  a  point  fourteen  miles  from  Cadiz  in  the  homeward 
direction.  Here  there  is  a  sudden  change  from  yellow- 
green  to  a  bright  emerald-green,  and  accompanying 
the  change  a  sudden  fall  in  the  quantity  of  suspended 
matter.  Between  Cape  St.  Mary  and  Cape  St.  Vin- 
cent the  water  changes  to  the  deepest  indigo,  a  fur- 
ther diminution  of  the  suspended  matter  being  the 
concomitant  phenomenon. 

We  now  reach  the  remarkable  group  of  rocks  called 
the  Burlings,  and  find  the  water  between  the  shore  and 
the  rocks  a  strong  green;  the  home  examination  shows 
it  to  be  thick  with  fine  matter.  Fifteen  or  twenty 
miles  beyond  the  Burlings  we  come  again  into  indigo 
water,  from  which  the  suspended  matter  has  in  great 
part  disappeared.  Off  Cape  Finisterre,  about  the  place 
where  the  '  Captain '  went  down,  the  water  becomes 
green,  and  the  home  examination  pronounces  it  to  be 
thicker.  Then  we  enter  the  Bay  of  Biscay,  where  the 
indigo  resumes  its  power,  and  where  the  home  exam- 
ination shows  the  greatly  augmented  purity  of  the 


170  FRAGMENTS    OF    SCIENCE. 

water.  A  second  specimen  of  water,  taken  from  the 
Bay  of  Biscay,  held  in  suspension  fine  particles  of  a 
peculiar  kind;  the  size  of  them  was  such  as  to  render 
the  water  richly  iridescent.  It  showed  itself  green, 
blue,  or  salmon-coloured,  according  to  the  direction  of 
the  line  of  vision.  Finally,  we  come  to  our  last  two 
bottles,  the  one  taken  opposite  St.  Catherine's  light- 
house, in  the  Isle  of  Wight,  the  other  at  Spithead.  The 
sea  at  both  these  places  was  green,  and  both  specimens, 
as  might  be  expected,  were  pronounced  by  the  home 
examination  to  be  thick  with  suspended  matter. 

Two  distinct  series  of  observations  are  here  referred 
to — the  one  consisting  of  direct  observations  of  the 
colour  of  the  sea,  conducted  during  the  voyage  from 
Gibraltar  to  Portsmouth:  the  other  carried  out  in  the 
laboratory  of  the  Eoyal  Institution.  And  here  it  is  to 
be  noted  that  in  the  home  examination  I  never  knew 
what  water  was  placed  in  my  hands.  The  labels,  with 
the  names  of  the  localities  written  upon  them,  had 
been  tied  up,  all  information  regarding  the  source  of 
the  water  being  thus  held  back.  The  bottles  were  sim- 
ply numbered,  and  not  till  all  of  them  had  been  ex- 
amined, and  described,  were  the  labels  opened,  and  the 
locality  and  sea-colour  corresponding  to  the  various 
specimens  ascertained.  The  home  observations,  there- 
fore, must  have  been  perfectly  unbiassed,  and  they 
clearly  establish  the  association  of  the  green  colour 
with  fine  suspended  matter,  and  of  the  ultramarine 
colour,  and  more  especially  of  the  black-indigo  hue  of 
the  Atlantic,  with  the  comparative  absence  of  such 
matter. 

So  much  for  mere  observation;  but  what  is  the 
cause  of  the  dark  hue  of  the  deep  ocean?  *  A  prelimi- 

*  A  note,  written  to  me  on  October  22,  by  my  friend  Canon 
Kingsley,  contains  the  following  reference  to  this  point :  '  I  have 


VOYAGE    TO   ALGERIA.  171 

nary  remark  or  two  will  clear  our  way  towards  an  ex- 
planation. Colour  resides  in  white  light,  appearing 
when  any  constituent  of  the  white  light  is  withdrawn. 
The  hue  of  a  purple  liquid,  for  example,  is  immedi- 
ately accounted  for  by  its  action  on  a  spectrum.  It 
cuts  out  the  yellow  and  green,  and  allows  the  red  and 
blue  to  pass  through.  The  blending  of  these  two 
colours  produces  the  purple.  But  while  such  a  liquid 
attacks  with  special  energy  the  yellow  and  green,  it 
enfeebles  the  whole  spectrum.  By  increasing  the  thick- 
ness of  the  stratum  we  may  absorb  the  whole  of  the 
light.  The  colour  of  a  blue  liquid  is  similarly  ac- 
counted for.  It  first  extinguishes  the  red;  then,  as  the 
thickness  augments,  it  attacks  the  orange,  yellow,  and 
green  in  succession;  the  blue  alone  finally  remaining. 
But  even  it  might  be  extinguished  by  a  sufficient  depth 
of  the  liquid. 

And  now  we  are  prepared  for  a  brief,  but  tolerably 
complete,  statement  of  that  action  of  sea-water  upon 
light,  to  which  it  owes  its  darkness.  The  spectrum 
embraces  three  classes  of  rays — the  thermal,  the  vis- 
ual, and  the  chemical.  These  divisions  overlap  each 
other;  the  thermal  rays  are  in  part  visual,  the  visual 
rays  in  part  chemical,  and  vice  versa.  The  vast  body 
of  thermal  rays  lie  beyond  the  red,  being  invisible. 
These  rays  are  attacked  with  exceeding  energy  by  water. 
They  are  absorbed  close  to  the  surface  of  the  sea,  and 
are  the  great  agents  in  evaporation.  At  the  same  time 
the  whole  spectrum  suffers  enfeeblement;  water  at- 
tacks all  its  rays,  but  with  different  degrees  of  energy. 

never  seen  the  Lake  of  Geneva,  but  I  thought  of  the  brilliant 
dazzling  dark  blue  of  the  mid-Atlantic  under  the  sunlight,  and 
its  black-blue  under  cloud,  both  so  solid  that  one  might  leap  off 
the  sponson  on  to  it  without  fear ;  this  was  to  me  the  most  won- 
derful thing  which  I  saw  on  my  voyages  to  and  from  the  West 
Indies.' 


172  FRAGMENTS    OP    SCIENCE. 

Of  the  visual  rays,  the  red  are  first  extinguished.  As 
the  solar  beam  plunges  deeper  into  the  sea,  orange  fol- 
lows red,  yellow  follows  orange,  green  follows  yellow, 
and  the  various  shades  of  blue,  where  the  water  is  deep 
enough,  follows  green.  Absolute  extinction  of  the 
solar  beam  would  be  the  consequence  if  the  water  were 
deep  and  uniform.  If  it  contained  no  suspended  mat- 
ter, such  water  would  be  as  black  as  ink.  A  reflected 
glimmer  of  ordinary  light  would  reach  us  from  its 
surface,  as  it  would  from  the  surface  of  actual  ink; 
but  no  light,  hence  no  colour,  would  reach  us  from  the 
body  of  the  water. 

In  very  clear  and  deep  sea-water  this  condition  is 
approximately  fulfilled,  and  hence  the  extraordinary 
darkness  of  such  water.  The  indigo,  already  referred 
to,  is,  I  believe,  to  be  ascribed  in  part  to  the  suspended 
matter,  which  is  never  absent,  even  in  the  purest  natu- 
ral water;  and  in  part  to  the  slight  reflection  of  the 
light  from  the  limiting  surfaces  of  strata  of  different 
densities.  A  modicum  of  light  is  thus  thrown  back 
to  the  eye,  before  the  depth  necessary  to  absolute  ex- 
tinction has  been  attained.  An  effect  precisely  similar 
occurs  under  the  moraines  of  glaciers.  The  ice  here  is 
exceptionally  compact,  and,  owing  to  the  absence  of 
the  internal  scattering  common  in  bubbled  ice,  the 
light  plunges  into  the  mass,  where  it  is  extinguished, 
the  perfectly  clear  ice  presenting  an  appearance  of 
pitchy  blackness.* 

The  green  colour  of  the  sea  has  now  to  be  accounted 
for;  and  here,  again,  let  us  fall  back  upon  the  sure 
basis  of  experiment.  A  strong  white  dinner-plate  had 
a  lead  weight  securely  fastened  to  it.  Fifty  or  sixty 
yards  of  strong  hempen  line  were  attached  to  the  plate. 

*  I  learn  from  a  correspondent  that  certain  Welsh  tarns, 
which  are  reputed  bottomless,  have  this  inky  hue. 


VOYAGE    TO   ALGERIA.  173 

My  assistant,  Thorogood,  occupied  a  boat,  fastened  as 
usual  to  the  davits  of  the  '  Urgent/  while  I  occupied 
a  second  boat  nearer  the  stern  of  the  ship.  He  cast  the 
plate  as  a  mariner  heaves  the  lead,  and  by  the  time  it 
reached  me  it  had  sunk  a  considerable  depth  in  the 
water.  In  all  cases  the  hue  of  this  plate  was  green. 
Even  when  the  sea  was  of  the  darkest  indigo,  the  green 
was  vivid  and  pronounced.  I  could  notice  the  gradual 
deepening  of  the  colour  as  the  plate  sank,  but  at  its 
greatest  depth,  even  in  indigo  water,  the  colour  was 
still  a  blue-green.* 

Other  observations  confirmed  this  one.  The 
'  Urgent '  is  a  screw  steamer,  and  right  over  the  blades 
of  the  screw  was  an  orifice  called  the  screw-well, 
through  which  one  could  look  from  the  poop  down 
upon  the  screw.  The  surface-glimmer,  which  so  pes- 
ters the  eye,  was  here  in  a  great  measure  removed. 
Midway  down,  a  plank  crossed  the  screw-well  from  side 
to  side;  on  this  I  placed  myself  and  observed  the 
action  of  the  screw  underneath.  The  eye  was  rendered 
sensitive  by  the  moderation  of  the  light;  and,  to  re- 
move still  further  all  disturbing  causes,  Lieutenant 
Walton  had  a  sail  and  tarpaulin  thrown  over  the  mouth 
of  the  well.  Underneath  this  I  perched  myself  on  the 
plank  and  watched  the  screw.  In  an  indigo  sea  the 
play  of  colour  was  indescribably  beautiful,  and  the 
contrast  between  the  water,  which  had  the  screw- 
blades,  and  that  which  had  the  bottom  of  the  ocean, 
as  a  background,  was  extraordinary.  The  one  was  of 
the  most  brilliant  green,  the  other  of  the  deepest  ultra- 
marine. The  surface  of  the  water  above  the  screw- 
blade  was  always  ruffled.  Liquid  lenses  were  thus 
formed,  by  which  the  coloured  light  was  withdrawn 

*  In  no  case,  of  course,  is  the  green  pure,  but  a  mixture  of 
green  and  blue. 


174  FRAGMENTS    OF    SCIENCE. 

from  some  places  and  concentrated  upon  others,  the 
water  flashing  with  metallic  lustre.  The  screw-blades 
in  this  case  played  the  part  of  the  dinner-plate  in  the 
former  case,  and  there  were  other  instances  of  a  similar 
kind.  The  white  bellies  of  porpoises  showed  the  green 
hue,  varying  in  intensity  as  the  creatures  swung  to 
and  fro  between  the  surface  and  the  deeper  water. 
Foam,  at  a  certain  depth  below  the  surface,  was  also 
green.  In  a  rough  sea  the  light  which  penetrated  the 
summit  of  a  wave  sometimes  reached  the  eye,  a  beauti- 
ful green  cap  being  thus  placed  upon  the  wave,  even 
in  indigo  water. 

But  how  is  this  colour  to  be  connected  with  the 
suspended  particles?  Thus.  Take  the  dinner-plate 
which  showed  so  brilliant  a  green  when  thrown  into 
indigo  water.  Suppose  it  to  diminish  in  size,  until  it 
reaches  an  almost  microscopic  magnitude.  It  would 
still  behave  substantially  as  the  larger  plate,  sending 
to  the  eye  its  modicum  of  green  light.  If  the  plate, 
instead  of  being  a  large  coherent  mass,  were  ground  to 
a  powder  sufficiently  fine,  and  in  this  condition  dif- 
fused through  the  clear  sea-water,  it  would  also  send 
green  light  to  the  eye.  In  fact,  the  suspended  particles 
which  the  home  examination  reveals,  act  in  all  essen- 
tial particulars  like  the  plate,  or  like  the  screw-blades, 
or  like  the  foam,  or  like  the  bellies  of  the  porpoises. 
Thus  I  think  the  greenness  of  the  sea  is  physically  con- 
nected with  the  matter  which  it  holds  in  suspension. 

We  reached  Portsmouth  on  January  5,  1871.  Then 
ended  a  voyage  which,  though  its  main  object  was  not 
realized,  has  left  behind  it  pleasant  memories,  both  of 
the  aspects  of  nature  and  the  kindliness  of  men. 


VII. 

NIAGARA* 

IT  is  one  of  the  disadvantages  of  reading  books  about 
natural  scenery  that  they  fill  the  mind  with  pic- 
tures, often  exaggerated,  often  distorted,  often  blurred, 
and,  even  when  well  drawn,  injurious  to  the  freshness 
of  first  impressions.  Such  has  been  the  fate  of  most 
of  us  with  regard  to  the  Falls  of  Niagara.  There  was 
little  accuracy  in  the  estimates  of  the  first  observers 
of  the  cataract.  Startled  by  an  exhibition  of  power 
so  novel  and  so  grand,  emotion  leaped  beyond  the  con- 
trol of  the  judgment,  and  gave  currency  to  notions 
which  have  often  led  to  disappointment. 

A  record  of  a  voyage  in  1535  by  a  French  mariner 
named  Jacques  Cartier,  contains,  it  is  said,  the  first 
printed  allusion  to  Niagara.  In  1603  the  first  map  of 
the  district  was  constructed  by  a  Frenchman  named 
Champlain.  In  1648  the  Jesuit  Rageneau,  in  a  letter 
to  his  superior  at  Paris,  mentions  Niagara  as  '  a  cata- 
ract of  frightful  height/  f  In  the  winter  of  1678  and 
1679  the  cataract  was  visited  by  Father  Hennepin,  and 
described  in  a  book  dedicated  '  to  the  King  of  Great 
Britain.'  He  gives  a  drawing  of  the  waterfall,  which 

*  A  Discourse  delivered  at  the  Royal  Institution  of  Great 
Britain,  April  4, 1878. 

f  From  an  interesting  little  book  presented  to  me  at  Brooklyn 
by  its  author,  Mr.  Holly,  some  of  these  data  are  derived :  Henne- 
pin, Kalm,  Bakewell,  Lyell,  Hall,  and  others  I  have  myself  con- 
sulted. 

175 


176  FRAGMENTS    OF    SCIENCE. 

shows  that  serious  changes  have  taken  place  since  his 
time.  He  describes  it  as  'a  great  and  prodigious 
cadence  of  water,  to  which  the  universe  does  not  offer 
a  parallel.'  The  height  of  the  fall,  according  to  Hen- 
nepin,  was  more  than  600  feet.  '  The  waters/  he  says, 
'  which  fall  from  this  great  precipice  do  foam  and  boil 
in  the  most  astonishing  manner,  making  a  noise  more 
terrible  than  that  of  thunder.  When  the  wind  blows 
to  the  south  its  frightful  roaring  may  be  heard  for 
more  than  fifteen  leagues.'  The  Baron  la  Hontan,  who 
visited  Niagara  in  1687,  makes  the  height  800  feet. 
In  1721  Charlevois,  in  a  letter  to  Madame  de  Mainte- 
non,  after  referring  to  the  exaggerations  of  his  prede- 
cessors, thus  states  the  result  of  his  own  observations: 
'  For  my  part,  after  examining  it  on  all  sides,  I  am 
inclined  to  think  that  we  cannot  allow  it  less  than  140 
or  150  feet,' — a  remarkably  close  estimate.  At  that 
time,  viz.  a  hundred  and  fifty  years  ago,  it  had  the 
shape  of  a  horseshoe,  and  reasons  will  subsequently  be 
given  for  holding  that  this  has  been  always  the  form  of 
the  cataract,  from  its  origin  to  its  present  site. 

As  regards  the  noise  of  the  fall,  Charlevois  declares 
the  accounts  of  his  predecessors,  which,  I  may  say,  are 
repeated  to  the  present  hour,  to  be  altogether  extrava- 
gant. He  is  perfectly  right.  The  thunders  of  Niagara 
are  formidable  enough  to  those  who  really  seek  them 
at  the  base  of  the  Horeshoe  Fall;  but  on  the  banks  of 
the  river,  and  particularly  above  the  fall,  its  silence, 
rather  than  its  noise,  is  surprising.  This  arises,  in 
part,  from  the  lack  of  resonance;  the  surrounding 
country  being  flat,  and  therefore  furnishing  no  echoing 
surfaces  to  reinforce  the  shock  of  the  water.  The 
resonance  from  the  surrounding  rocks  causes  the  Swiss 
Reuss  at  the  Devil's  Bridge,  when  full,  to  thunder 
more  loudly  than  the  Niagara. 


NIAGARA.  177 

On  Friday,  November  1,  1872,  just  before  reaching 
the  village  of  Niagara  Falls,  I  caught,  from  the  railway 
train,  my  first  glimpse  of  the  smoke  of  the  cataract. 
Immediately  after  my  arrival  I  went  with  a  friend  to 
the  northern  end  of  the  American  Fall.  It  may  be 
that  my  mood  at  the  time  toned  down  the  impression 
produced  by  the  first  aspect  of  this  grand  cascade;  but 
I  felt  nothing  like  disappointment,  knowing,  from  old 
experience,  that  time  and  close  acquaintanceship,  the 
gradual  interweaving  of  mind  and  nature,  must  power- 
fully influence  my  final  estimate  of  the  scene.  After 
dinner  we  crossed  to  Goat  Island,  and,  turning  to  the 
right,  reached  the  southern  end  of  the  American  Fall. 
The  river  is  here  studded  with  small  islands.  Crossing 
a  wooden  bridge  to  Luna  Island,  and  clasping  a  tree 
which  grows  near  its  edge,  I  looked  long  at  the  cata- 
ract, which  here  shoots  down  the  precipice  like  an 
avalanche  of  foam.  It  grew  in  power  and  beauty.  The 
channel  spanned  by  the  wooden  bridge  was  deep,  and 
the  river  there  doubled  over  the  edge  of  the  precipice, 
like  the  swell  of  a  muscle,  unbroken.  The  ledge  here 
overhangs,  the  water  being  poured  out  far  beyond  the 
base  of  the  precipice.  A  space,  called  the  Cave  of  the 
Winds,  is  thus  enclosed  between  the  wall  of  rock  and 
the  falling  water. 

Goat  Island  ends  in  a  sheer  dry  precipice,  which 
connects  the  American  and  Horseshoe  Falls.  Midway 
between  both  is  a  wooden  hut,  the  residence  of  the 
guide  to  the  Cave  of  the  Winds,  and  from  the  hut  a 
winding  staircase,  called  Biddle's  Stair,  descends  to 
the  base  of  the  precipice.  On  the  evening  of  my  ar- 
rival I  went  down  this  stair,  and  wandered  along  the 
bottom  of  the  cliff.  One  well-known  factor  in  the 
formation  and  retreat  of  the  cataract  was  immediately 
observed.  A  thick  layer  of  limestone  formed  the  upper 


178  FRAGMENTS    OF    SCIENCE. 

portion  of  the  cliff.  This  rested  upon  a  bed  of  soft 
shale,  which  extended  round  the  base  of  the  cataract. 
The  violent  recoil  of  .the  water  against  this  yielding 
substance  crumbles  it  away,  undermining  the  ledge 
above,  which,  unsupported,  eventually  breaks  off,  and 
produces  the  observed  recession. 

At  the  southern  extremity  of  the  Horseshoe  is  a 
promontory,  formed  by  the  doubling  back  of  the  gorge 
excavated  T>y  the  cataract,  and  into  which  it  plunges. 
On  the  promontory  stands  a  stone  building,  called  the 
Terrapin  Tower,  the  door  of  which  had  been  nailed  up 
because  of  the  decay  of  the  staircase  within  it.  Through 
the  kindness  of  Mr.  Townsend,  the  superintendent  of 
Goat  Island,  the  door  was  opened  for  me.  From  this 
tower,  at  all  hours  of  the  day,  and  at  some  hours  of  the 
night,  I  watched  and  listened  to  the  Horseshoe  Fall. 
The  river  here  is  evidently  much  deeper  than  the 
American  branch;  and  instead  of  bursting  into  foam 
where  it  quits  the  ledge,  it  bends  solidly  over,  and  falls 
in  a  continuous  layer  of  the  most  vivid  green.  The 
tint  is  not  uniform;  long  stripes  of  deeper  hue  alter- 
nating with  bands  of  brighter  colour.  Close  to  the 
ledge  over  which  the  water  rolls,  foam  is  generated, 
the  light  falling  upon  which,  and  flashing  back  from 
it,  is  sifted  in  its  passage  to  and  fro,  and  changed  from 
white  to  emerald-green.  Heaps  of  superficial  foam  are 
also  formed  at  intervals  along  the  ledge,  and  are  im- 
mediately drawn  into  long  white  striae.*  Lower  down, 
the  surface,  shaken  by  the  reaction  from  below,  in- 
cessantly rustles  into  whiteness.  The  descent  finally 
resolves  itself  into  a  rhythm,  the  water  reaching  the 
bottom  of  the  fall  in  periodic  gushes.  Nor  is  the 

*  The  direction  of  the  wind  with  reference  to  the  course  of  a 
ship  may  be  inferred  with  accuracy  from  the  foam-streaks  on  the 
surface  of  the  sea. 


NIAGARA..  179 

spray  uniformly  diffused  through  the  air,  but  is  wafted 
through  it  in  successive  veils  of  gauze-like  texture. 
From  all  this  it  is  evident  that  beauty  is  not  absent 
from  the  Horseshoe  Fall,  but  majesty  is  its  chief  at- 
tribute. The  plunge  of  the  water  is  not  wild,  but 
deliberate,  vast,  and  fascinating.  From  the  Terrapin 
Tower,  the  adjacent  arm  of  the  Horseshoe  is  seen  pro- 
jected against  the  opposite  one,  midway  down;  to  the 
imagination,  therefore,  is  left  the  picturing  of  the 
gulf  into  which  the  cataract  plunges. 

The  delight  which  natural  scenery  produces  in 
some  minds  is  difficult  to  explain,  and  the  conduct 
which  it  prompts  can  hardly  be  fairly  criticised  by 
those  who  have  never  experienced  it.  It  seems  to  me 
a  deduction  from  the  completeness  of  the  celebrated 
Thomas  Young,  that  he  was  unable  to  appreciate  natu- 
ral scenery.  *  He  had  really/  says  Dean  Peacock,  '  no 
taste  for  life  in  the  country;  he  was  one  of  those  who 
thought  that  no  one  who  was  able  to  live  in  London 
would  be  content  to  live  elsewhere.'  Well,  Dr.  Young, 
like  Dr.  Johnson,  had  a  right  to  his  delights;  but  I  can 
understand  a  hesitation  to  accept  them,  high  as  they 
were,  to  the  exclusion  of 

That  o'erflowing  joy  which  Nature  yields 
To  her  true  lovers. 

To  all  who  are  of  this  mind,  the  strengthening  of 
desire  on  my  part  to  see  and  know  Niagara  Falls,  as 
far  as  it  is  possible  for  them  to  be  seen  and  known, 
will  be  intelligible. 

On  the  first  evening  of  my  visit,  I  met,  at  the  head 
of  Biddle's  Stair,  the  guide  to  the  Cave  of  the  Winds. 
He  was  in  the  prime  of  manhood — large,  well  built, 
firm  and  pleasant  in  mouth  and  eye.  My  interest  in 
the  scene  stirred  up  his,  and  made  him  communicative. 


180  FRAGMENTS    OF    SCIENCE. 

Turning  to  a  photograph,  he  described,  by  reference  to 
it,  a  feat  which  he  had  accomplished  some  time  pre- 
viously, and  which  had  brought  him  almost  under  the 
green  water  of  the  Horseshoe  Fall.  '  Can  you  lead  me 
there  to-morrow?'  I  asked.  He  eyed  me  enquiringly, 
weighing,  perhaps,  the  chances  of  a  man  of  light  build, 
and  with  grey  in  his  whiskers,  in  such  an  undertaking. 
*  I  wish/  I  added,  '  to  see  as  much  of  the  fall  as  can  be 
seen,  and  where  you  lead  I  will  endeavour  to  follow/ 
His  scrutiny  relaxed  into  a  smile,  and  he  said,  '  Very 
well;  I  shall  be  ready  for  you  to-morrow.' 

On  the  morrow,  accordingly,  I  came.  In  the  hut 
at  the  head  of  Biddle's  Stair  I  stripped  wholly,  and 
re-dressed  according  to  instructions, — drawing  on  two 
pairs  of  woollen  pantaloons,  three  woollen  jackets,  two 
pairs  of  socks,  and  a  pair  of  felt  shoes.  Even  if  wet, 
my  guide  assured  me  that  the  clothes  would  keep  me 
from  being  chilled;  and  he  was  right.  A  suit  and 
hood  of  yellow  oilcloth  covered  all.  Most  laudable  pre- 
cautions were  taken  by  the  young  assistant  who  helped 
to  dress  me  to  keep  the  water  out;  but  his  devices 
broke  down  immediately  when  severely  tested. 

We  descended  the  stair;  the  handle  of  a  pitchfork 
doing,  in  my  case,  the  duty  of  an  alpenstock.  At  the 
bottom,  the  guide  enquired  whether  we  should  go  first 
to  the  Cave  of  the  Winds,  or  to  the  Horseshoe,  remark- 
ing that  the  latter  would  try  us  most.  I  decided  on 
getting  the  roughest  done  first,  and  he  turned  to  the 
left  over  the -stones.  They  were  sharp  and  trying.  The 
base  of  the  first  portion  of  the  cataract  is  covered 
with  huge  boulders,  obviously  the  ruins  of  the  lime- 
stone ledge  above.  The  water  does  not  distribute  itself 
uniformly  among  these,  but  seeks  out  channels  through 
which  it  pours  torrentially.  We  passed  some  of  these 
with  wetted  feet,  but  without  difficulty.  At  length 


NIAGARA.  181 

we  came  to  the  side  of  a  more  formidable  current. 
My  guide  walked  along  its  edge  until  he  reached  its 
least  turbulent  portion.  Halting,  he  said,  '  This  is 
our  greatest  difficulty;  if  we  can  cross  here,  we  shall 
get  far  towards  the  Horseshoe.' 

He  waded  in.  It  evidently  required  all  his  strength 
to  steady  him.  The  water  rose  above  his  loins,  and  it 
foamed  still  higher.  He  had  to  search  for  footing, 
amid  unseen  boulders,  against  which  the  torrent  rose 
violently.  He  struggled  and  swayed,  but  he  struggled 
successfully,  and  finally  reached  the  shallower  water  at 
the  other  side.  Stretching  out  his  arm,  he  said  to  me, 
'  Now  come  on/  I  looked  down  the  torrent,  as  it 
rushed  to  the  river  below,  which  was  seething  with  the 
tumult  of  the  cataract.  De  Saussure  recommended 
the  inspection  of  Alpine  dangers,  with  the  view  of 
making  them  familiar  to  the  eye  before  they  are  en- 
countered; and  it  is  a  wholesome  custom  in  places  of 
difficulty  to  put  the  possibility  of  an  accident  clearly 
before  the  mind,  and  to  decide  beforehand  what  ought 
to  be  done  should  the  accident  occur.  Thus  wound 
up  in  the  present  instance,  I  entered  the  water.  Even 
where  it  was  not  more  than  knee-deep,  its  power  was 
manifest.  As  it  rose  around  me,  I  sought  to  split  the 
torrent  by  presenting  a  side  to  it;  but  the  insecurity 
of  the  footing  enabled  it  to  grasp  my  loins,  twist  me 
fairly  round,  and  bring  its  impetus  to  bear  upon  my 
back.  Further  struggle  was  impossible;  and  feeling 
my  balance  hopelessly  gone,  I  turned,  flung  myself 
toward  the  bank  just  quitted,  and  was  instantly,  as 
expected,  swept  into  shallower  water. 

The  oilcloth  covering  was  a  great  incumbrance;  it 
had  been  made  for  a  much  stouter  man,  and,  standing 
upright  after  my  submersion,  my  legs  occupied  the 
centre  of  two  bags  of  water.  My  guide  exhorted  me  to 


182  FKAGMENTS    OF    SCIENCE. 

try  again.  Prudence  was  at  my  elbow,  whispering 
dissuasion;  but,  taking  everything  into  account,  it 
appeared  more  immoral  to  retreat  than  to  proceed. 
Instructed  by  the  first  misadventure,  I  once  more 
entered  the  stream.  Had  the  alpenstock  been  of  iron 
it  might  have  helped  me;  but,  as  it  was,  the  tendency 
of  the  water  to  sweep  it  out  of  my  hands  rendered  it 
worse  than  useless.  I,  however,  clung  to  it  by  habit. 
Again  the  torrent  rose,  and  again  I  wavered;  but,  by 
keeping  the  left  hip  well  against  it,  I  remained  up- 
right, and  at  length  grasped  the  hand  of  my  leader  at 
the  other  side.  He  laughed  pleasantly.  The  first  vic- 
tory was  gained,  and  he  enjoyed  it.  '  No  traveller,'  he 
said,  '  was  ever  here  before.'  Soon  afterwards,  by 
trusting  to  a  piece  of  drift-wood  which  seemed  firm,  I 
was  again  taken  off  my  feet,  but  was  immediately 
caught  by  a  protruding  rock. 

We  clambered  over  the  boulders  towards  the  thick- 
est spray,  which  soon  became  so  weighty  as  to  cause  us 
to  stagger  under  its  shock.  For  the  most  part  nothing 
could  be  seen;  we  were  in  the  midst  of  bewildering 
tumult,  lashed  by  the  water,  which  sounded  at  times 
like  the  cracking  of  innumerable  whips.  Underneath 
this  was  the  deep  resonant  roar  of  the  cataract.  I 
tried  to  shield  my  eyes  with  my  hands,  and  look  up- 
wards; but  the  defence  was  useless.  The  guide  con- 
tinued to  move  on,  but  at  a  certain  place  he  halted, 
desiring  me  to  take  shelter  in  his  lee,  and  observe 
the  cataract.  The  spray  did  not  come  so  much  from 
the  upper  ledge,  as  from  the  rebound  of  the  shattered 
water  when  it  struck  the  bottom.  Hence  the  eyes 
could  be  protected  from  the  blinding  shock  of  the 
spray,  while  the  line  of  vision  to  the  upper  ledges 
remained  to  some  extent  clear.  On  looking  upwards 
over  the  guide's  shoulder  I  could  see  the  water  bending 


NIAGARA.  183 

over  the  ledge,  while  the  Terrapin  Tower  loomed  fit- 
fully through  the  intermittent  spray-gusts.  We  were 
right  under  the  tower.  A  little  farther  on  the  cataract, 
after  its  first  plunge,  hit  a  protuberance  some  way 
down,  and  flew  from  it  in  a  prodigious  burst  of  spray; 
through  this  we  staggered.  We  rounded  the  promon- 
tory on  which  the  Terrapin  Tower  stands,  and  moved, 
amid  the  wildest  commotion,  along  the  arm  of  the 
Horseshoe,  until  the  boulders  failed  us,  and  the  cata- 
ract fell  into  the  profound  gorge  of  the  Niagara  River. 
Here  the  guide  sheltered  me  again,  and  desired  me 
to  look  up;  I  did  so,  and  could  see,  as  before,  the 
green  gleam  of  the  mighty  curve  sweeping  over  the 
upper  ledge,  and  the  fitful  plunge  of  the  water,  as 
the  spray  between  us  and  it  alternately  gathered  and 
disappeared.  An  eminent  friend  of  mine  often  speaks 
of  the  mistake  of  those  physicians  who  regard  man's 
ailments  as  purely  chemical,  to  be  met  by  chemical 
remedies  only.  He  contends  for  the  psychological 
element  of  cure.  By  agreeable  emotions,  he  says, 
nervous  currents  are  liberated  which  stimulate  blood, 
brain,  and  viscera.  The  influence  rained  from  ladies' 
eyes  enables  my  friend  to  thrive  on  dishes  which  would 
kill  him  if  eaten  alone.  A  sanative  effect  of  the  same 
order  I  experienced  amid  the  spray  and  thunder  of 
Niagara.  Quickened  by  the  emotions  there  aroused, 
the  blood  sped  exultingly  through  the  arteries,  abolish- 
ing introspection,  clearing  the  heart  of  all  bitterness, 
and  enabling  one  to  think  with  tolerance,  if  not  with 
tenderness,  on  the  most  relentless  and  unreasonable 
foe.  Apart  from  its  scientific  value,  and  purely  as  a 
moral  agent,  the  play  was  worth  the  candle.  My  com- 
panion knew  no  more  of  me  than  that  I  enjoyed  the 
wildness  of  the  scene;  but  as  I  bent  in  the  shelter  of 
his  large  frame  he  said,  '  I  should  like  to  see  you  at- 
18 


184  FKAGMENTS    OF    SCIENCE. 

tempting  to  describe  all  this/  He  rightly  thought  it 
indescribable.  The  name  of  this  gallant  fellow  was 
Thomas  Conroy. 

We  returned,  clambering  at  intervals  up  and  down, 
so  as  to  catch  glimpses  of  the  most  impressive  portions 
of  the  cataract.  We  passed  under  ledges  formed  by 
tabular  masses  of  limestone,  and  through  some  curious 
openings  formed  by  the  falling  together  of  the  sum- 
mits of  the  rocks.  At  length  we  found  ourselves  be- 
side our  enemy  of  the  morning.  Conroy  halted  for  a 
minute  or  two,  scanning  the  torrent  thoughtfully.  I 
said  that,  as  a  guide,  he  ought  to  have  a  rope  in  such  a 
place;  but  he  retorted  that,  as  no  traveller  had  ever 
thought  of  coming  there,  he  did  not  see  the  necessity 
of  keeping  a  rope.  He  waded  in.  The  struggle  to  keep 
himself  erect  was  evident  enough;  he  swayed,  but  re- 
covered himself  again  and  again.  At  length  he  slipped, 
gave  way,  did  as  I  had  done,  threw  himself  towards 
the  bank,  and  was  swept  into  the  shallows.  Standing 
in  the  stream  near  its  edge,  he  stretched  his  arm  to- 
wards me.  I  retained  the  pitchfork  handle,  for  it  had 
been  useful  among  the  boulders.  By  wading  some 
way  in,  the  staff  could  be  made  to  reach  him,  and  I 
proposed  his.  seizing  it.  *  If  you  are  sure,'  he  replied, 
'  that,  in  case  of  giving  way,  you  can  maintain  your 
grasp,  then  I  will  certainly  hold  you.'  Eemarking  that 
he  might  count  on  this,  I  waded  in,  and  stretched  the 
staff  to  my  companion.  It  was  firmly  grasped  by  both 
of  us.  Thus  helped,  though  its  onset  was  strong,  I 
moved  safely  across  the  torrent.  All  danger  ended 
here.  We  afterwards  roamed  sociably  among  the  tor- 
rents and  boulders  below  the  Cave  of  the  Winds.  The 
rocks  were  covered  with  organic  slime,  which  could 
not  have  been  walked  over  with  bare  feet,  but  the  felt 
shoes  effectually  prevented  slipping.  We  reached  the 


NIAGARA.  185 

cave  and  entered  it,  first  by  a  wooden  way  carried  over 
the  boulders,  and  then  along  a  narrow  ledge,  to  the 
point  eaten  deepest  into  the  shale.  When  the  wind  is 
from  the  south,  the  falling  water,  I  am  told,  can  be 
seen  tranquilly  from  this  spot;  but  when  we  were 
there,  a  blinding  hurricane  of  spray  was  whirled 
against  us.  On  the  evening  of  the  same  day,  I  went 
behind  the  water  on  the  Canada  side,  which,  after  the 
experiences  of  the  morning,  struck  me  as  an  imposture. 

Still  even  this  latter  is  exciting  to  some  nerves. 
Its  effect  upon  himself  is  thus  vividly  described  by 
Mr.  Bakewell,  jun.:  '  On  turning  a  sharp  angle  of  the 
rock,  a  sudden  gust  of  wind  met  us,  coming  from  the 
hollow  between  the  fall  and  the  rock,  which  drove  the 
spray  directly  in  our  faces,  with  such  force  that  in  an 
instant  we  were  wet  through.  When  in  the  midst  of 
this  shower-bath  the  shock  took  away  my  breath:  I 
turned  back  and  scrambled  over  the  loose  stones  to 
escape  the  conflict.  The  guide  soon  followed,  and  told 
me  that  I  had  passed  the  worst  part.  With  that  as- 
surance I  made  a  second  attempt;  but  so  wild  and 
disordered  was  my  imagination  that  when  I  had 
reached  half  way  I  could  bear  it  no  longer/  * 

To  complete  my  knowledge  I  desired  to  see  the  fall 
from  the  river  below  it,  and  long  negotiations  were 
necessary  to  secure  the  means  of  doing  so.  The  only 
boat  fit  for  the  undertaking  had  been  laid  up  for  the 
winter;  but  this  difficulty,  through  the  kind  interven- 
tion of  Mr.  Townsend,  was  overcome.  The  main  one 
was  to  secure  oarsmen  sufficiently  strong  and  skilful  to 
urge  the  boat  where  I  wished  it  to  be  taken.  The  son 
of  the  owner  of  the  boat,  a  finely-built  young  fellow, 
but  only  twenty,  and  therefore  not  sufficiently  hard- 
ened, was  willing  to  go;  and  up  the  river,  it  was  stated, 
»  '  Mag.  of  Nat.  Hist.,'  1830,  pp.  121, 123. 


186  FRAGMENTS    OF    SCIENCE. 

there  lived  another  man  who  could  do  anything  with 
the  boat  which  strength  and  daring  could  accomplish. 
He  came.  His  figure  and  expression  of  face  certainly 
indicated  extraordinary  firmness  and  power.  On  Tues- 
day, November  5,  we  started,  each  of  us  being  clad  in 
oilcloth.  The  elder  oarsman  at  once  assumed  a  tone 
of  authority  over  his  companion,  and  struck  imme- 
diately in  amid  the  breakers  below  the  American  Fall. 
He  hugged  the  cross  freshets  instead  of  striking  out 
into  the  smoother  water.  I  asked  him  why  he  did  so, 
and  he  replied  that  they  were  directed  outwards,  not 
downwards.  The  struggle,  however,  to  prevent  the 
bow  of  the  boat  from  being  turned  by  them,  was  often 
very  severe. 

The  spray  was  in  general  blinding,  but  at  times  it 
disappeared  and  yielded  noble  views  of  the  fall.  The 
edge  of  the  cataract  is  crimped  by  indentations  which 
exalt  its  beauty.  Here  and  there,  a  little  below  the 
highest  ledge,  a  secondary  one  juts  out;  the  water 
strikes  it  and  bursts  from  it  in  huge  protuberant  masses 
of  foam  and  spray.  We  passed  Goat  Island,  came  to 
the  Horseshoe,  and  worked  for  a  time  along  its  base, 
the  boulders  over  which  Conroy  and  myself  had  scram- 
bled a  few  days  previously  lying  between  us  and  the 
cataract.  A  rock  was  before  us,  concealed  and  revealed 
at  intervals,  as  the  waves  passed  over  it.  Our  leader 
tried  to  get  above  this  rock,  first  on  the  outside  of  it. 
The  water,  however,  was  here  in  violent  motion.  The 
men  struggled  fiercely,  the  older  one  ringing  out  an 
incessant  peal  of  command  and  exhortation  to  the 
younger.  As  we  were  just  clearing  the  rock,  the  bow 
came  obliquely  to  the  surge;  the  boat  was  turned  sud- 
denly round  and  shot  with  astonishing  rapidity  down 
the  river.  The  men  returned  to  the  charge,  now 
trying  to  get  up  between  the  half-concealed  rock  and 


NIAGARA.  187 

the  boulders  to  the  left.  But  the  torrent  set  in  strong- 
ly through  this  channel.  The  tugging  was  quick  and 
violent,  but  we  made  little  way.  At  length,  seizing  a 
rope,  the  principal  oarsman  made  a  desperate  attempt 
to  get  upon  one  of  the  boulders,  hoping  to  be  able  to 
drag  the  boat  through  the  channel;  but  it  bumped  so 
violently  against  the  rock,  that  the  man  flung  himself 
back  and  relinquished  the  attempt. 

We  returned  along  the  base  of  the  American  Fall, 
running  in  and  out  among  the  currents  which  rushed 
from  it  laterally  into  the  river.  Seen  from  below  the 
American  Fall  is  certainly  exquisitely  beautiful,  but  it 
is  a  mere  frill  of  adornment  to  its  nobler  neighbour  the 
Horseshoe.  At  times  we  took  to  the  river,  from  the 
centre  of  which  the  Horseshoe  Fall  appeared  especially 
magnificent.  A  streak  of  cloud  across  the  neck  of 
Mont  Blanc  can  double  its  apparent  height,  so  here 
the  green  summit  of  the  cataract  shining  above  the 
smoke  of  spray  appeared  lifted  to  an  extraordinary 
elevation.  Had  Hennepin  and  La  Hontan  seen  the 
fall  from  this  position,  their  estimates  of  the  height 
would  have  been  perfectly  excusable. 

From  a  point  a  little  way  below  the  American  Fall, 
a  ferry  crosses  the  river,  in  summer,  to  the  Canadian 
side.  Below  the  ferry  is  a  suspension  bridge  for  car- 
riages and  foot-passengers,  and  a  mile  or  two  lower 
down  is  the  railway  suspension  bridge.  Between  ferry 
and  bridge  the  river  Niagara  flows  unruffled;  but  at 
the  suspension  bridge  the  bed  steepens  and  the  river 
quickens  its  motion.  Lower  down  the  gorge  narrows, 
and  the  rapidity  and  turbulence  increase.  At  the 
place  called  the  *  Whirlpool  Rapids '  I  estimated  the 
width  of  the  river  at  300  feet,  an  estimate  confirmed 
by  the  dwellers  on  the  spot.  When  it  is  remembered 


188  FRAGMENTS    OP    SCIENCE. 

that  the  drainage  of  nearly  half  a  continent  is  com- 
pressed into  this  space,  the  impetuosity  of  the  river's 
rush  may  be  imagined. .  Had  it  not  been  for  Mr.  Bier- 
stadt,  the  distinguished  photographer  of  Niagara,  I 
should  have  quitted  the  place  without  seeing  these 
rapids;  for  this,  and  for  his  agreeable  company  to  the 
spot,  I  have  to  thank  him.  From  the  edge  of  the  cliff 
above  the  rapids,  we  descended,  a  little,  I  confess,  to  a 
climber's  disgust,  in  an  '  elevator/  because  the  effects 
are  best  seen  from  the  water  level. 

Two  kinds  of  motion  are  here  obviously  active,  a 
motion  of  translation  and  a  motion  of  undulation — the 
race  of  the  river  through  its  gorge,  and  the  great  waves 
generated  by  its  collision  with,  and  rebound  from,  the 
obstacles  in  its  way.  In  the  middle  of  the  river  the 
rush  and  tossing  are  most  violent;  at  all  events,  the 
impetuous  force  of  the  individual  waves  is  here  most 
strikingly  displayed.  Yast  pyramidal  heaps  leap  inces- 
santly from  the  river,  some  of  them  with  such  energy 
as  to  jerk  their  summits  into  the  air,  where  they  hang 
momentarily  suspended  in  crowds  of  liquid  spherules. 
The  sun  shone  for  a  few  minutes.  At  times  the  wind, 
coming  up  the  river,  searched  and  sifted  the  spray, 
carrying  away  the  lighter  drops,  and  leaving  the 
heavier  ones  behind.  Wafted  in  the  proper  direction, 
rainbows  appeared  and  disappeared  fitfully  in  the  light- 
er mist.  In  other  directions  the  common  gleam  of  the 
sunshine  from  the  waves  and  their  shattered  crests  was 
exquisitely  beautiful.  The  complexity  of  the  action 
was  still  further  illustrated  by  the  fact,  that  in  some 
cases,  as  if  by  the  exercise  of  a  local  explosive  force, 
the  drops  were  shot  radially  from  a  particular  centre, 
forming  around  it  a  kind  of  halo. 

The  first  impression,  and,  indeed,  the  current  ex- 
planation of  these  rapids  is,  that  the  central  bed  of 


NIAGARA.  189 

the  river  is  cumbered  with  large  boulders,  and  that  the 
jostling,  tossing,  and  wild  leaping  of  the  water  there, 
are  due  to  its  impact  against  these  obstacles.  I  doubt 
this  explanation.  At  all  events,  there  is  another  suffi- 
cient reason  to  be  taken  into  account.  Boulders  de- 
rived from  the  adjacent  cliffs  visibly  cumber  the  sides 
of  the  river.  Against  these  the  water  rises  and  sinks 
rhythmically  but  violently,  large  waves  being  thus  pro- 
duced. On  the  generation  of  each  wave,  there  is  an 
immediate  compounding  of  the  wave-motion  with  the 
river-motion.  The  ridges,  which  in  still  water  would 
proceed  in  circular  curves  round  the  centre  of  disturb- 
ance, cross  the  river  obliquely,  and  the  result  is  that 
at  the  centre  waves  commingle,  which  have  really  been 
generated  at  the  sides.  In  the  first  instance,  we  had  a 
composition  of  wave-motion  with  river-motion;  here 
we  have  the  coalescence  of  waves  with  waves.  Where 
crest  and  furrow  cross  each  other,  the  motion  is  an- 
nulled; where  furrow  and  furrow  cross,  the  river  is 
ploughed  to  a  greater  depth;  and  where  crest  and  crest 
aid  each  other,  we  have  that  astonishing  leap  of  the 
water  which  breaks  the  cohesion  of  the  crests,  and 
tosses  them  shattered  into  the  air.  From  the  water 
level  the  cause  of  the  action  is  not  so  easily  seen;  but 
from  the  summit  of  the  cliff  the  lateral  generation  of 
the  waves,  and  their  propagation  to  the  centre,  are 
perfectly  obvious.  If  this  explanation  be  correct,  the 
phenomena  observed  at  the  Whirlpool  Rapids  form  one 
of  the  grandest  illustrations  of  the  principle  of  inter- 
ference. The  Nile  '  cataract/  Mr.  Huxley  informs  me, 
offers  more  moderate  examples  of  the  same  action. 

At  some  distance  below  the  Whirlpool  Rapids  we 
have  the  celebrated  whirlpool  itself.  Here  the  river 
makes  a  sudden  bend  to  the  north-east,  forming  nearly 
a  right  angle  with  its  previous  direction.  The  water 


190  FRAGMENTS    OF    SCIENCE. 

strikes  the  concave  bank  with  great  force,  and  scoops  it 
incessantly  away.  A  vast  basin  has  been  thus  formed, 
in  which  the  sweep  of  the  river  prolongs  itself  in 
gyratory  currents.  Bodies  and  trees  which  have  come 
over  the  falls,  are  stated  to  circulate  here  for  days  with- 
out finding  the  outlet.  From  various  points  of  the 
cliffs  above,  this  is  curiously  hidden.  The  rush  of  the 
river  into  the  whirlpool  is  obvious  enough;  and  though 
you  imagine  the  outlet  must  be  visible,  if  one  existed, 
you  cannot  find  it.  Turning,  however,  round  the  bend 
of  the  precipice  to  the  north-east,  the  outlet  comes  into 
view. 

The  Niagara  season  was  over;  the  chatter  of  sight- 
seers had  ceased,  and  the  scene  presented  itself  as  one 
of  holy  seclusion  and  beauty.  I  went  down  to  the 
river's  edge,  where  the  weird  loneliness  seemed  to  in~ 
crease.  The  basin  is  enclosed  by  high  and  almost  pre- 
cipitous banks — covered,  at  the  time,  with  russet 
woods.  A  kind  of  mystery  attaches  itself  to  gyrating 
water,  due  perhaps  to  the  fact  that  we  are  to  some 
extent  ignorant  of  the  direction  of  its  force.  It  is  said 
that  at  certain  points  of  the  whirlpool,  pine-trees  are 
sucked  down,  to  be  ejected  mysteriously  elsewhere. 
The  water  is  of  the  brightest  emerald-green.  The 
gorge  through  which  it  escapes  is  narrow,  and  the 
motion  of  the  river  swift  though  silent.  The  surface  is 
steeply  inclined,  but  it  is  perfectly  unbroken.  There 
are  no  lateral  waves,  no  ripples  with  their  breaking- 
bubbles  to  raise  a  murmur;  while  the  depth  is  here  too 
great  to  allow  the  inequality  of  the  bed  to  ruffle  the 
surface.  Nothing  can  be  more  beautiful  than  this 
sloping  liquid  mirror  formed  by  the  Niagara,  in  sliding 
from  the  whirlpool. 

The  green  colour  is,  I  think,  correctly  accounted 
for  in  the  last  Fragment.  While  crossing  the  Atlantic 


NIAGARA.  191 

in  1872-73  I  had  frequent  opportunities  of  testing  the 
explanation  there  given.  Looked  properly  down  upon, 
there  are  portions  of  the  ocean  to  which  we  should 
hardly  ascribe  a  trace  of  blue;  at  the  most,  a  mere  hint 
of  indigo  reaches  the  eye.  The  water,  indeed,  is  prac- 
tically black,  and  this  is  an  indication  both  of  its  depth 
and  of  its  freedom  from  mechanically  suspended  mat- 
ter. In  small  thicknesses  water  is  sensibly  transparent 
to  all  kinds  of  light;  but,  as  the  thickness  increases, 
the  rays  of  low  refrangibility  are  first  absorbed,  and 
after  them  the  other  rays.  Where,  therefore,  the  water 
is  very  deep  and  very  pure,  all  the  colours  are  absorbed, 
and  such  water  ought  to  appear  black,  as  no  light  is 
sent  from  its  interior  to  the  eye.  The  approximation 
of  the  Atlantic  Ocean  to  this  condition  is  an  indication 
of  its  extreme  purity. 

Throw  a  white  pebble  into  such  water;  as  it  sinks 
it  becomes  greener  and  greener,  and,  before  it  disap- 
pears, it  reaches  a  vivid  blue-green.  Break  such  a 
pebble  into  fragments,  each  of  these  will  behave  like 
the  unbroken  mass;  grind  the  pebble  to  powder,  every 
particle  will  yield  its  modicum  of  green;  and  if  the 
particles  be  so  fine  as  to  remain  suspended  in  the  water, 
the  scattered  light  will  be  a  uniform  green.  Hence  the 
greenness  of  shoal  water.  You  go  to  bed  with  the 
black  Atlantic  around  you.  You  rise  in  the  morning, 
find  it  a  vivid  green,  and  correctly  infer  that  you  are 
crossing  the  bank  of  Newfoundland.  Such  water  is 
found  charged  with  fine  matter  in  a  state  of  mechan- 
ical suspension.  The  light  from  the  bottom  may  some- 
times come  into  play,  but  it  is  not  necessary.  A  storm 
can  render  the  water  muddy,  by  rendering  the  particles 
too  numerous  and  gross.  Such  a  case  occurred  towards 
the  close  of  my  visit  to  Niagara.  There  had  been  rain 
and  storm  in  the  upper  lake-regions,  and  the  quantity 


192  FEAGMENTS    OF    SCIENCE. 

of  suspended  matter  brought  down  quite  extinguished 
the  fascinating  green  of  the  Horseshoe. 

Nothing  can  be  more  superb  than  the  green  of  the 
Atlantic  waves,  when  the  circumstances  are  favourable 
to  the  exhibition  of  the  colour.  As  long  as  a  wave 
remains  unbroken  no  colour  appears;  but  when  the 
foam  just  doubles  over  the  crest,  like  an  Alpine  snow- 
cornice,  under  the  cornice  we  often  see  a  display  of  the 
most  exquisite  green.  It  is  metallic  in  its  brilliancy. 
But  the  foam  is  necessary  to  its  production.  The  foam 
is  first  illuminated,  and  it  scatters  the  light  in  all  di- 
rections; the  light  which  passes  through  the  higher 
portion  of  the  wave  alone  reaches  the  eye,  and  gives  to 
that  portion  its  matchless  colour.  The  folding  of  the 
wave,  producing  as  it  does  a  series  of  longitudinal  pro- 
tuberances and  furrows  which  act  like  cylindrical 
lenses,  introduces  variations  in  the  intensity  of  the 
light,  and  materially  enhances  its  beauty. 

We  have  now  to  consider  the  genesis  and  proximate 
destiny  of  the  Falls  of  Niagara.  We  may  open  our 
way  to  this  subject  by  a  few  preliminary  remarks  upon 
erosion.  Time  and  intensity  are  the  main  factors  of 
geologic  change,  and  they  are  in  a  certain  sense  con- 
vertible. A  feeble  force  acting  through  long  periods, 
and  an  intense  force  acting  through  short  ones,  may 
produce  approximately  the  same  results.  To  Dr. 
Hooker  I  have  been  indebted  for  some  specimens  of 
stones,  the  first  examples  of  which  were  picked  up  by 
Mr.  Hackworth  on  the  shores  of  Lyell's  Bay,  near  Well- 
ington, in  New  Zealand.  They  were  described  by  Mr. 
Travers  in  the  '  Transactions  of  the  New  Zealand  In- 
stitute.' Unacquainted  with  their  origin,  you  would 
certainly  ascribe  their  forms  to  human  workmanship. 
They  resemble  knives  and  spear-heads,  being  apparent- 


NIAGARA.  193 

ly  chiselled  off  into  facets,  with  as  much  attention  to 
symmetry  as  if  a  tool,  guided  by  human  intelligence, 
had  passed  over  them.  But  no  human  instrument  has 
been  brought  to  bear  upon  these  stones.  They  have 
been  wrought  into  their  present  shape  by  the  wind- 
blown sand  of  LyelTs  Bay.  Two  winds  are  dominant 
here,  and  they  in  succession  urged  the  sand  against 
opposite  sides  of  the  stone;  every  little  particle  of  sand 
chipped  away  its  infinitesimal  bit  of  stone,  and  in  the 
end  sculptured  these  singular  forms.* 

The  Sphynx  of  Egypt  is  nearly  covered  up  by  the 
sand  of  the  desert.  The  neck  of  the  Sphynx  is  partly 
cut  across,  not,  as  I  am  assured  by  Mr.  Huxley,  by 
ordinary  weathering,  but  by  the  eroding  action  of  the 
fine  sand  blown  against  it.  In  these  cases  Nature 
furnishes  us  with  hints  which  may  be  taken  advantage 
of  in  art;  and  this  action  of  sand  has  been  recently 
turned  to  extraordinary  account  in  the  United  States. 
When  in  Boston,  I  was  taken  by  my  courteous  and  help- 

•  '  These  stones,  which  have  a  strong  resemblance  to  works  of 
human  art,  occur  in  great  abundance,  and  of  various  sizes,  from 
half-an-inch  to  several  inches  in  length.  A  large  number  were 
exhibited  showing  the  various  forms,  which  are  those  of  wedges, 
knives,  arrow-heads,  &c.,  and  all  with  sharp  cutting  edges. 

'  Mr.  Travers  explained  that,  notwithstanding  their  artificial 
appearance,  these  stones  were  formed  by  the  cutting  action  of 
the  wind-driven  sand,  as  it  passed  to  and  fro  over  an  exposed 
boulder-bank.  He  gave  a  minute  account  of  the  manner  in 
which  the  varieties  of  form  are  produced,  and  referred  to  the 
effect  which  the  erosive  action  thus  indicated  would  have  on 
railway  and  other  works  executed  on  sandy  tracts. 

4  Dr.  Hector  stated  that  although,  as  a  group,  the  specimens 
on  the  table  could  not  well  be  mistaken  for  artificial  productions, 
still  the  forms  are  so  peculiar,  and  the  edges,  in  a  few  of  them, 
so  perfect,  that  if  they  were  discovered  associated  with  human 
works,  there  is  no  doubt  that  they  would  have  been  referred  to 
the  so-called  "  stone  period."  ' — Extracted  from  the  Minutes  of 
the  Wellington  Philosophical  Society,  February  9, 1869. 


194  FKAGMENTS    OF    SCIENCE. 

ful  friend,  Mr.  Josiah  Quincey,  to  see  the  action  of  the 
sand-blast.  A  kind  of  hopper  containing  fine  silicious 
sand  was  connected  with  a  reservoir  of  compressed  air, 
the  pressure  being  variable  at  pleasure.  The  hopper 
ended  in  a  long  slit,  from  which  the  sand  was  blown. 
A  plate  of  glass  was  placed  beneath  this  slit,  and  caused 
to  pass  slowly  under  it;  it  came  out  perfectly  depol- 
ished,  with  a  bright  opalescent  glimmer,  such  as  could 
only  be  produced  by  the  most  careful  grinding.  Every 
little  particle  of  sand  urged  against  the  glass,  having 
all  its  energy  concentrated  on  the  point  of  impact, 
formed  there  a  little  pit,  the  depolished  surface  consist- 
ing of  innumerable  hollows  of  this  description. 

But  this  was  not  all.  By  protecting  certain  por- 
tions of  the  surface,  and  exposing  others,  figures  and 
tracery  of  any  required  form  could  be  etched  upon  the 
glass.  The  figures  of  open  iron-work  could  be  thus 
copied;  while  wire-gauze  placed  over  the  glass  pro- 
duced a  reticulated  pattern.  But  it  required  no  such 
resisting  substance  as  iron  to  shelter  the  glass.  The 
patterns  of  the  finest  lace  could  be  thus  reproduced; 
the  delicate  filaments  of  the  lace  itself  offering  a  suf- 
ficient protection.  All  these  effects  have  been  obtained 
with  a  simple  model  of  the  sand-blast  devised  by  my 
assistant.  A  fraction  of  a  minute  suffices  to  etch  upon 
glass  a  rich  and  beautiful  lace  pattern.  Any  yielding 
substance  may  be  employed  to  protect  the  glass.  By 
diffusing  the  shock  of  the  particle,  such  substances 
practically  destroy  the  local  erosive  power.  The  hand 
can  bear,  without  inconvenience,  a  sand-shower  which 
would  pulverise  glass.  Etchings  executed  on  glass  with 
suitable  kinds  of  ink  are  accurately  worked  out  by  the 
sand-blast.  In  fact,  within  certain  limits,  the  harder 
the  surface,  the  greater  is  the  concentration  of  the 
shock,  and  the  more  effectual  is  the  erosion.  It  is  not 


NIAGARA.  195 

necessary  that  the  sand  should  be  the  harder  substance 
of  the  two;  corundum,  for  example,  is  much  harder 
than  quartz;  still,  quartz-sand  can  not  only  depolish, 
but  actually  blow  a  hole  through  a  plate  of  corundum. 
Nay,  glass  may  be  depolished  by  the  impact  of  fine 
shot;  the  grains  in  this  case  bruising  the  glass,  before 
they  have  time  to  flatten  and  turn  their  energy  into 
heat. 

And  here,  in  passing,  we  may  tie  together  one  or 
two  apparently  unrelated  facts.  Supposing  you  turn 
on,  at  the  lower  part  of  a  house,  a  cock  which  is  fed  by 
a  pipe  from  a  cistern  at  the  top  of  the  house,  the  col- 
umn .of  water,  from  the  cistern  downwards,  is  set  in 
motion.  By  turning  off  the  cock,  this  motion  is 
stopped;  and  when  the  turning  off  is  very  sudden,  the 
pipe,  if  not  strong,  may  be  burst  by  the  internal  im- 
pact of  the  water.  By  distributing  the  turning  of  the 
cock  over  half  a  second  of  time,  the  shock  and  danger 
of  rupture  may  be  entirely  avoided.  We  have  here 
an  example  of  the  concentration  of  energy  in  lime. 
The  sand-blast  illustrates  the  concentration  of  energy 
in  space.  The  action  of  flint  and  steel  is  an  illustra- 
tion of  the  same  principle.  The  heat  required  to  gen- 
erate the  spark  is  intense;  and  the  mechanical  action, 
being  moderate,  must,  to  produce  fire,  be  in  the  high- 
est degree  concentrated.  This  concentration  is  secured 
by  the  collision  of  hard  substances.  Calc-spar  will  not 
supply  the  place  of  flint,  nor  lead  the  place  of  steel,  in 
the  production  of  fire  by  collision.  With  the  softer 
substances,  the  total  heat  produced  may  be  greater 
than  with  the  hard  ones,  but,  to  produce  the  spark, 
the  heat  must  be  intensely  localised. 

We  can,  however,  go  far  beyond  the  mere  depolish- 
ing  of  glass;  indeed  I  have  already  said  that  quartz- 
sand  can  wear  a  hole  through  corundum.  This  leads 


196  FKAGMENTS    OF    SCIENCE. 

me  to  express  my  acknowledgments  to  General  Tilgh- 
man,*  who  is  the  inventor  of  the  sand-blast.  To  his 
spontaneous  kindness  I  am  indebted  for  some  beautiful 
illustrations  of  his  process.  In  one  thick  plate  of  glass 
a  figure  has  been  worked  out  to  a  depth  of  f  ths  of  an 
inch.  A  second  plate,  £ths  of  an  inch  thick,  is  entirely 
perforated.  In  a  circular  plate  of  marble,  nearly  half 
an  inch  thick,  open  work  of  most  intricate  and  elab- 
orate description  has  been  executed.  It  would  prob- 
ably take  many  days  to  perform  this  work  by  any 
ordinary  process;  with  the  sand-blast  it  was  accom- 
plished in  an  hour.  So  much  for  the  strength  of  the 
blast;  its  delicacy  is  illustrated  by  this  beautiful  ex- 
ample of  line  engraving,  etched  on  glass  by  means  of 
the  blast. 

This  power  of  erosion,  so  strikingly  displayed  when 
sand  is  urged  by  air,  renders  us  better  able  to  conceive 
its  action  when  urged  by  water.  The  erosive  power  of 
a  river  is  vastly  augmented  by  the  solid  matter  carried 
along  with  it.  Sand  or  pebbles,  caught  in  a  river  vor- 
tex, can  wear  away  the  hardest  rock;  *  potholes '  and 
deep  cylindrical  shafts  being  thus  produced.  An  ex- 
traordinary instance  of  this  kind  of  erosion  is  to  be 
seen  in  the  Val  Tournanche,  above  the  village  of  this 
name.  The  gorge  at  Handeck  has  been  thus  cut  out. 
Such  waterfalls  were  once  frequent  in  the  valleys  of 
Switzerland;  for  hardly  any  valley  is  without  one  or 
more  transverse  barriers  of  resisting  material,  over 

*  The  absorbent  power,  if  I  may  use  the  phrase,  exerted  by 
the  industrial  arts  in  the  United  States,  is  forcibly  illustrated  by 
the  rapid  transfer  of  men  like  Mr.  Tilghman  from  the  life  of  the 
soldier  to  that  of  the  civilian.  General  McClellan,  now  a  civil 
engineer,  whom  I  had  the  honour  of  frequently  meeting  in  New 
York,  is  a  most  eminent  example  of  the  same  kind.  At  the  end 
of  the  war,  indeed,  a  million  and  a  half  of  men  were  thus  drawn 
in  an  astonishingly  short  time,  from  military  to  civil  life. 


NIAGARA.  197 

which  the  river  flowing  through  the  valley  once  fell  as 
a  cataract.  Near  Pontresina,  in  the  Engadin,  there  is 
such  a  case;  a  hard  gneiss  there  worn  away  to  form  a 
gorge,  through  which  the  river  from  the  Morteratsch 
glacier  rushes.  The  barrier  of  the  Kirchet  above  Mey- 
ringen  is  also  a  case  in  point.  Behind  it  was  a  lake,  de- 
rived from  the  glacier  of  the  Aar,  and  over  the  barrier 
the  lake  poured  its  excess  of  water.  Here  the  rock, 
being  limestone,  was  in  part  dissolved;  but  added  to 
this  we  had  the  action  of  the  sand  and  gravel  carried 
along  by  the  water,  which,  on  striking  the  rock, 
chipped  it  away  like  the  particles  of  the  sand-blast. 
Thus,  by  solution  and  mechanical  erosion,  the  great 
chasm  of  the  Finsteraarschlucht  was  formed.  It  is 
demonstrable  that  the  water  which  flows  at  the  bottoms 
of  such  deep  fissures  once  flowed  at  the  level  of  their 
present  edges,  and  tumbled  down  the  lower  faces  of 
the  barriers.  Almost  every  valley  in  Switzerland  fur- 
nishes examples  of  this  kind;  the  untenable  hypothesis 
of  earthquakes,  once  so  readily  resorted  to  in  account- 
ing for  these  gorges,  being  now  for  the  most  part  aban- 
doned. To  produce  the  Canons  of  Western  America, 
no  other  cause  is  needed  than  the  integration  of  effects 
individually  infinitesimal. 

And  now  we  come  to  Niagara.  Soon  after  Euro- 
peans had  taken  possession  of  the  country,  the  con- 
viction appears  to  have  arisen  that  the  deep  channel  of 
the  river  Niagara  below  the  falls  had  been  excavated 
by  the  cataract.  In  Mr.  Bakewell's  *  Introduction  to 
Geology,'  the  prevalence  of  this  belief  has  been  referred 
to.  It  is  expressed  thus  by  Professor  Joseph  Henry  in 
the  '  Transactions  of  the  Albany  Institute:'*  *  In  view- 
ing the  position  of  the  falls,  and  the  features  of  the 
country  round,  it  is  impossible  not  to  be  impressed  with 
*  Quoted  by  Bakewell. 


198  FRAGMENTS    OF    SCIENCE. 

the  idea  that  this  great  natural  raceway  has  been 
formed  by  the  continued  action  of  the  irresistible  Niag- 
ara, and  that  the  falls,  beginning  at  Lewiston,  have,  in 
the  course  of  ages,  worn  back  the  rocky  strata  to  their 
present  site.'  The  same  view  is  advocated  by  Sir 
Charles  Lyell,  by  Mr.  Hall,  by  M.  Agassiz,  by  Professor 
Ramsay,  indeed  by  most  of  those  who  have  inspected 
the  place. 

A  connected  image  of  the  origin  and  progress  of 
the  cataract  is  easily  obtained.  Walking  northward 
from  the  village  of  Niagara  Falls  by  the  side  of  the 
river,  we  have  to  our  left  the  deep  and  comparatively 
narrow  gorge,  through  which  the  Niagara  flows.  The 
bounding  cliffs  of  this  gorge  are  from  300  to  350  feet 
high.  We  reached  the  whirlpool,  trend  to  the  north- 
east, and  after  a  little  time  gradually  resume  our  north- 
ward course.  Finally,  at  about  seven  miles  from  the 
present  falls,  we  come  to  the  edge  of  a  declivity,  which 
informs  us  that  we  have  been  hitherto  walking  on 
table-land.  At  some  hundreds  of  feet  below  us  is  a 
comparatively  level  plain,  which  stretches  to  Lake  On- 
tario. The  declivity  marks  the  end  of  the  precipitous 
gorge  of  the  Niagara.  Here  the  river  escapes  from  its 
steep  mural  boundaries,  and  in  a  widened  bed  pursues 
its  way  to  the  lake  which  finally  receives  its  waters. 

The  fact  that  in  historic  times,  even  within  the 
memory  of  man,  the  fall  has  sensibly  receded,  prompts 
the  question,  How  far  has  this  recession  gone?  At 
what  point  did  the  ledge  which  thus  continually  creeps 
backwards  begin  its  retrograde  course?  To  minds 
disciplined  in  such  researches  the  answer  has  been,  and 
will  be — At  the  precipitous  declivity  which  crossed  the 
Niagara  from  Lewiston  on  the  American  to  Queenston 
on  the  Canadian  side.  Over  this  transverse  barrier  the 
united  affluents  of  all  the  upper  lakes  once  poured  their 


NIAGARA.  199 

waters,  and  here  the  work  of  erosion  began.  The  dam, 
moreover,  was  demonstrably  of  sufficient  height  to 
cause  the  river  above  it  to  submerge  Goat  Island;  and 
this  would  perfectly  account  for  the  finding  by  Sir 
Charles  Lyell,  Mr.  Hall,  and  others,  in  the  sand  and 
gravel  of  the  island,  the  same  fluviatile  shells  as  are 
now  found  in  the  Niagara  River  higher  up.  It  would 
also  account  for  those  deposits  along  the  sides  of  the 
river,  the  discovery  of  which  enabled  Lyell,  Hall,  and 
Ramsay  to  reduce  to  demonstration  the  popular  be- 
lief that  the  Niagara  once  flowed  through  a  shallow 
valley. 

The  physics  of  the  problem  of  excavation,  which  I 
made  clear  to  my  mind  before  quitting  Niagara,  are  re- 
vealed by  a  close  inspection  of  the  present  Horseshoe 
Fall.  We  see  evidently  that  the  greatest  weight  of 
water  bends  over  the  very  apex  of  the  Horseshoe.  In 
a  passage  in  his  excellent  chapter  on  Niagara  Falls,  Mr. 
Hall  alludes  to  this  fact.  Here  we  have  the  most  copi- 
ous and  the  most  violent  whirling  of  the  shattered 
liquid;  here  the  most  powerful  eddies  recoil  against 
the  shale.  From  this  portion  of  the  fall,  indeed,  the 
spray  sometimes  rises  without  solution  of  continuity  to 
the  region  of  clouds,  becoming  gradually  more  attenu- 
ated, and  passing  finally  through  the  condition  of  true 
cloud  into  invisible  vapour,  which  is  sometimes  repre- 
cipitated  higher  up.  All  the  phenomena  pointdistinctly 
to  the  centre  of  the  river  as  the  place  of  greatest  me- 
chanical energy,  and  fron\  the  centre  the  vigour  of  the 
fall  gradually  dies  away  towards  the  sides.  The  Horse- 
shoe form,  with  the  concavity  facing  downwards,  is  an 
obvious  and  necessary  consequence  of  this  action. 
Right  along  the  middle  of  the  river  the  apex  of  the 
curve  pushes  its  way  backwards,  cutting  along  the 
centre  a  deep  and  comparatively  narrow  groove,  and 
14 


200  FRAGMENTS    OF    SCIENCE. 

draining  the  sides  as  it  passes  them.*  Hence  the  re- 
markable discrepancy  between  the  widths  of  the  Niag- 
ara above  and  below  the  Horseshoe.  All  along  its 
course,  from  Lewiston  Heights  to  its  present  position, 
the  form  of  the  fall  was  probably  that  of  a  horseshoe; 
for  this  is  merely  the  expression  of  the  greater  depth, 
and  consequently  greater  excavating  power,  of  the 
centre  of  the  river.  The  gorge,  moreover,  varies  in 
width,  as  the  depth  of  the  centre  of  the  ancient  river 
varied,  being  narrowest  where  that  depth  was  greatest. 
The  vast  comparative  erosive  energy  of  the  Horse- 
shoe Fall  comes  strikingly  into  view  when  it  and  the 
American  Fall  are  compared  together.  The  American 
branch  of  the  river  is  cut  at  a  right  angle  by  the  gorge 
of  the  Niagara.  Here  the  Horseshoe  Fall  was  the  real 
excavator.  It  cut  the  rock,  and  formed  the  precipice, 
over  which  the  American  Fall  tumbles.  But  since  its 
formation,  the  erosive  action  of  the  American  Fall  has 
been  almost  nil,  while  the  Horseshoe  has  cut  its  way 
for  500  yards  across  the  end  of  Goat  Island,  and  is  now 
doubling  back  to  excavate  its  channel  parallel  to  the 
length  of  the  island.  This  point,  which  impressed  me 
forcibly,  has  not,  I  have  just  learned,  escaped  the  acute 
observation  of  Professor  Eamsay.f  The  river  bends; 
the  Horseshoe  immediately  accommodates  itself  to  the 
bending,  and  will  follow  implicitly  the  direction  of 
the  deepest  water  in  the  upper  stream.  The  flexures 

*  In  the  discourse  the  excavation  of  the  centre  and  drainage 
of  the  sides  action  was  illustrated  by  a  model  devised  by  my  as- 
sistant, Mr.  John  Cottrell. 

f  His  words  are :  '  Where  the  body  of  water  is  small  in  the 
American  Fall,  the  edge  has  only  receded  a  few  yards  (where 
most  eroded)  during  the  time  that  the  Canadian  Fall  has  receded 
from  the  north  corner  of  Goat  Island  to  the  innermost  curve  of 
the  Horseshoe  Fall.' — Quarterly  Journal  of  Geological  Society, 
May,  1859. 


NIAGARA.  201 

of  the  gorge  are  determined  by  those  of  the  river  chan- 
nel above  it.  Were  the  Niagara  centre  above  the  fall 
sinuous,  the  gorge  would  obediently  follow  its  sinuosi- 
ties. Once  suggested,  no  doubt  geographers  will  be 
able  to  point  out  many  examples  of  this  action.  The 
Zambesi  is  thought  to  present  a  great  difficulty  to  the 
erosion  theory,  because  of  the  sinuosity  of  the  chasm 
below  the  Victoria  Falls.  But,  assuming  the  basalt  to 
be  of  tolerably  uniform  texture,  had  the  river  been  ex- 
amined before  the  formation  of  this  sinuous  channel, 
the  present  zigzag  course  of  the  gorge  below  the  fall 
could,  I  am  persuaded,  have  been  predicted,  while  the 
sounding  of  the  present  river  would  enable  us  to  pre- 
dict the  course  to  be  pursued  by  the  erosion  in  the 
future. 

But  not  only  has  the  Niagara  River  cut  the  gorge; 
it  has  carried  away  the  chips  of  its  own  workshop.  The 
shale,  being  probably  crumbled,  is  easily  carried  away. 
But  at  the  base  of  the  fall  we  find  the  huge  boulders 
already  described,  and  by  some  means  or  other  these 
are  removed  down  the  river.  The  ice  which  fills  the 
gorge  in  winter,  and  which  grapples  with  the  boulders, 
has  been  regarded  as  the  transporting  agent.  Probably 
it  is  so  to  some  extent.  But  erosion  acts  without  ceas- 
ing on  the  abutting  points  of  the  boulders,  thus  with- 
drawing their  support  and  urging  them  gradually  down 
the  river.  Solution  also  does  its  portion  of  the  work. 
That  solid  matter  is  carried  down  is  proved  by  the  dif- 
ference of  depths  between  the  Niagara  River  and  Lake 
Ontario,  where  the  river  enters  it.  The  depth  falls 
from  72  feet  to  20  feet,  in  consequence  of  the  deposi- 
tion of  solid  matter  caused  by  the  diminished  motion 
of  the  river.* 

*  Near  the  mouth  of  the  gorge  at  Qtieenston,  the  depth,  ac- 
cording to  the  Admiralty  Chart,  is  180  feet;  well  within  the- 
gorge  it  is  182  feet 


203 


FRAGMENTS    OF    SCIENCE. 

FIG.  4. 


NIAGARA.  203 

The  annexed  highly  instructive  map  has  been  re- 
duced from  one  published  in  Mr.  Hall's  '  Geology  of 
New  York.'  It  is  based  on  surveys  executed  in  1842, 
by  Messrs.  Gibson  and  Evershed.  The  ragged  edge  of 
the  American  Fall  north  of  Goat  Island  marks  the 
amount  of  erosion  which  it  has  been  able  to  accom- 
plish, while  the  Horseshoe  Fall  was  cutting  its  way 
southward  across  the  end  of  Goat  Island  to  its  present 
position.  The  American  Fall  is  168  feet  high,  a  preci- 
pice cut  down,  not  by  itself,  but  by  the  Horseshoe  Fall. 
The  latter  in  1842  was  159  feet  high,  and,  as  shown  by 
the  map,  is  already  turning  eastward,  to  excavate  its 
gorge  along  the  centre  of  the  upper  river.  P  is  the 
apex  of  the  Horseshoe,  and  T  marks  the  site  of  the 
Terrapin  Tower,  with  the  promontory  adjacent,  round 
which  I  was  conducted  by  Conroy.  Probably  since 
1842  the  Horseshoe  has  worked  back  beyond  the  posi- 
tion here  assigned  to  it. 

In  conclusion,  we  may  say  a  word  regarding  the 
proximate  future  of  Niagara.  At  the  rate  of  excava- 
tion assigned  to  it  by  Sir  Charles  Lyell,  namely,  a  foot 
a  year,  five  thousand  years  or  so  will  carry  the  Horse- 
shoe Fall  far  higher  than  Goat  Island.  As  the  gorge 
recedes  it  will  drain,  as  it  has  hitherto  done,  the  banks 
right  and  left  of  it,  thus  leaving  a  nearly  level  terrace 
between  Goat  Island  and  the  edge  of  the  gorge.  Higher 
up  it  will  totally  drain  the  American  branch  of  the 
river;  the  channel  of  which  in  due  time  will  become 
cultivable  land.  The  American  Fall  will  then  be  trans- 
formed into  a  dry  precipice,  forming  a  simple  continua- 
tion of  the  cliffy  boundary  of  the  Niagara  gorge.  At 
the  place  occupied  by  the  fall  at  this  moment  we  shall 
have  the  gorge  enclosing  a  right  angle,  a  second  whirl- 
pool being  the  consequence.  To  those  who  visit  Niag- 
ara a  few  millenniums  hence  I  leave  the  verification  of 


204  FRAGMENTS    OF    SCIENCE. 

this  prediction.  All  that  can  be  said  is,  that  if  the 
causes  now  in  action  continue  to  act,  it  will  prove  itself 
literally  true. 

POSTSCRIPT. 

A  year  or  so  after  I  had  quitted  the  United  States, 
a  man  sixty  years  of  age,  while  engaged  in  painting 
one  of  the  bridges  which  connect  Goat  Island  with  the 
Three  Sisters,  slipped  through  the  rails  of  the  bridge 
into  the  rapids,  and  was  carried  impetuously  towards 
the  Horseshoe  Fall.  He  was  urged  against  a  rock 
which  rose  above  the  water,  and  with  the  grasp  of 
desperation  he  clung  to  it.  The  population  of  the 
village  of  Niagara  Falls  was  soon  upon  the  island,  and 
ropes  Avere  brought,  but  there  was  none  to  use  them. 
In  the  midst  of  the  excitement,  a  tall  powerful  young 
fellow  was  observed  making  his  way  silently  through 
the  crowd.  He  reached  a  rope;  selected  from  the  by- 
standers a  number  of  men,  and  placed  one  end  of  the 
rope  in  their  hands.  The  other  end  he  fastened  round 
himself,  and  choosing  a  point  considerably  above  that 
to  which  the  man  clung,  he  plunged  into  the  rapids. 
He  Avas  carried  violently  doAvmvards,  but  he  caught 
the  rock,  secured  the  old  painter  and  saved  him.  NCAVS- 
papers  from  all  parts  of  the  Union  poured  in  upon 
me,  describing  this  gallant  act  of  my  guide  Conroy. 


VIII. 
THE  PARALLEL  ROADS  OF  GLEN  ROY* 

HE  first  published  allusion  to  the  Parallel  Roads 
-L  of  Glen  Roy  occurs  in  the  appendix  to  the  third 
volume  of  Pennant's  '  Tour  in  Scotland/  a  work  pub- 
lished in  1776.  'In  the  face  of  these  hills/  says  this 
writer, '  both  sides  of  the  glen,  there  are  three  roads  at 
small  distances  from  each  other  and  directly  opposite 
on  each  side.  These  roads  have  been  measured  in  the 
complete  parts  of  them,  and  found  to  be  26  paces  of  a 
man  5  feet  10  inches  high.  The  two  highest  are  pretty 
near  each  other,  about  50  yards,  and  the  lowest  double 
that  distance  from  the  nearest  to  it.  They  are  carried 
along  the  sides  of  the  glen  with  the  utmost  regularity, 
nearly  as  exact  as  drawn  with  a  line  of  rule  and  com- 
pass.' 

The  correct  heights  of  the  three  roads  of  Glen  Roy 
are  respectively  1150,  1070,  and  860  feet  above  the 
sea.  Hence  a  vertical  distance  of  80  feet  separates  the 
two  highest,  while  the  lowest  road  is  210  feet  below 
the  middle  one. 

These  'roads'  are  usually  shelves  or  terraces  formed 
in  the  yielding  drift  which  here  covers  the  slopes  of  the 
mountains.  They  are  all  sensibly  horizontal  and  there- 
fore parallel.  Pennant  accepted  as  reasonable  the  ex- 
planation of  them  given  by  the  country  people  in  his 

*  A  discourse  delivered  at  the  Royal  Institution  of  Great 
Britain  on  June  0, 1876. 

205 


206  FRAGMENTS    OF    SCIENCE. 

time.  They  thought  that  the  roads  '  were  designed  for 
the  chase,  and  that  the  terraces  were  made  after  the 
spots  were  cleared  in  lines  from  wood,  in  order  to  tempt 
the  animals  into  the  open  paths  after  they  were  rouzed, 
in  order  that  they  might  come  within  reach  of  the 
bowmen  who  might  conceal  themselves  in  the  woods 
above  and  below/ 

In  these  attempts  of  '  the  country  people '  we  have 
an  illustration  of  that  impulse  to  which  all  scientific 
knowledge  is  due — the  desire  to  know  the  causes  of 
things;  and  it  is  a  matter  of  surprise  that  in  the  case  of 
the  parallel  roads,  with  their  weird  appearance  chal- 
lenging enquiry,  this  impulse  did  not  make  itself  more 
rapidly  and  energetically  felt.  Their  remoteness  may 
perhaps  account  for  the  fact  that  until  the  year  1817 
no  systematic  description  of  them,  and  no  scientific 
attempt  at  an  explanation  of  them,  appeared.  In  that 
year  Dr.  MacCulloch,  who  was  then  President  of  the 
Geological  Society,  presented  to  that  Society  a  memoir, 
in  which  the  roads  were  discussed,  and  pronounced  to 
be  the  margins  of  lakes  once  embosomed  in  Glen  Koy. 
Why  there  should  be  three  roads,  or  why  the  lakes 
should  stand  at  these  particular  levels,  was  left  unex- 
plained. 

To  Dr.  MacCulloch  succeeded  a  man,  possibly  not 
so  learned  as  a  geologist,  but  obviously  fitted  by  nature 
to  grapple  with  her  facts  and  to  put  them  in  their 
proper  setting.  I  refer  to  Sir  Thomas  Dick-Lauder, 
who  presented  to  the  Royal  Society  of  Edinburgh,  on 
the  2nd  of  March,  1818,  his  paper  on  the  Parallel 
Roads  of  Glen  Roy.  In  looking  over  the  literature  of 
this  subject,  which  is  now  copious,  it  is  interesting  to 
observe  the  differentiation  of  minds,  and  to  single  out 
those  who  went  by  a  kind  of  instinct  to  the  core  of  the 
question,  from  those  who  erred  in  it,  or  who  learnedly 


THE    PARALLEL    ROADS    OF    GLEN    ROY.     207 

occupied  themselves  with  its  analogies,  adjuncts,  and 
details.  There  is  no  man,  in  my  opinion,  connected 
with  the  history  of  the  subject,  who  has  shown,  in 
relation  to  it,  this  spirit  of  penetration,  this  force  of 
scientific  insight,  more  conspicuously  than  Sir  Thomas 
Dick-Lauder.  Two  distinct  mental  processes  are  in- 
volved in  the  treatment  of  such  a  question.  Firstly, 
the  faithful  and  sufficient  observation  of  the  data;  and 
secondly,  that  higher  mental  process  in  which  the  con- 
structive imagination  comes  into  pla}r,  connecting  the 
separate  facts  of  observation  with  their  common  cause, 
and  weaving  them  into  an  organic  whole.  In  neither 
of  these  requirements  did  Sir  Thomas  Dick-Lauder  fail. 

Adjacent  to  Glen  Roy  is  a  valley  called  Glen  Gluoy, 
along  the  sides  of  which  ran  a  single  shelf,  or  terrace, 
formed  obviously  in  the  same  manner  as  the  parallel 
roads  of  Glen  Roy.  The  two  shelves  on  the  opposing 
sides  of  the  glen  were  at  precisely  the  same  level,  and 
Dick-Lauder  wished  to  see  whether,  and  how,  they 
became  united  at  the  head  of  the  glen.  He  followed 
the  shelves  into  the  recesses  of  the  mountains.  The 
bottom  of  the  valley,  as  it  rose,  came  ever  nearer  to 
them,  until  finally,  at  the  head  of  Glen  Gluoy,  he 
reached  a  col,  or  watershed,  of  precisely  the  same 
elevation  as  the  road  which  swept  round  the  glen. 

The  correct  height  of  this  col  is  1170  feet  above 
the  sea;  that  is  to  say,  20  feet  above  the  highest  road 
in  Glen  Roy. 

From  this  col  a  lateral  branch-valley — Glen  Turrit 
— led  down  to  Glen  Roy.  Our  explorer  descended  from 
the  col  to  the  highest  road  of  the  latter  glen,  and  pur- 
sued it  exactly  as  he  had  pursued  the  road  in  Glen 
Gluoy.  For  a  time  it  belted  the  mountain  sides  at  a 
considerable  height  above  the  bottom  of  the  valley;  but 
this  rose  as  he  proceeded,  coming  ever  nearer  to  the 


208 


FRAGMENTS    OF    SCIENCE. 


highest  shelf,  until  finally  he  reached  a  col,  or  water- 
shed, looking  into  Glen  Spey,  and  of  precisely  the  same 
elevation  as  the  highest  road  of  Glen  Roy. 


PARALLEL  ROADS  OF  GLEN  ROY. 
After  a  Sketch  by  SIB  THOMAS  DICK-LAUD EE. 

He  then  dropped  down  to  the  lowest  of  these  roads, 
and  followed  it  towards  the  mouth  of  the  glen.  Its 
elevation  above  the  bottom  of  the  valley  gradually  in- 
creased; not  because  the  shelf  rose,  but  because  it 
remained  level  while  the  valley  sloped  downwards.  He 
found  this  lowest  road  doubling  round  the  hills  at  the 
mouth  of  Glen  Eoy,  and  running  along  the  sides  of  the 
mountains  which  flank  Glen  Spean.  He  followed  it 
eastwards.  The  bottom  of  the  Spean  Valley,  like  the 
others,  gradually  rose,  and  therefore  gradually  ap- 
proached the  road  on  the  adjacent  mountain-side.  He 
came  to  Loch  Laggan,  the  surface  of  which  rose  almost 


THE  PARALLEL  ROADS  OF  GLEN  ROY.  209. 

to  the  level  of  the  road,  and  beyond  the  head  of  this 
lake  he  found,  as  in  the  other  two  cases,  a  col,  or  water- 
shed, at  Makul,  of  exactly  the  same  level  as  the  single 
road  in  Glen  Spean,  which,  it  will  be  remembered,  is  a 
continuation  of  the  lowest  road  in  Glen  Roy. 

Here  we  have  a  series  of  facts  of  obvious  signifi- 
cance as  regards  the  solution  of  this  problem.  The 
effort  of  the  mind  to  form  a  coherent  image  from  such 
facts  may  be  compared  with  the  effort  of  the  eyes  to 
cause  the  pictures  of  a  stereoscope  to  coalesce.  For  a 
time  we  exercise  a  certain  strain,  the  object  remaining 
vague  and  indistinct.  Suddenly  its  various  parts  seem 
to  run  together,  the  object  starting  forth  in  clear  and 
definite  relief.  Such,  I  take  it,  was  the  effect  of  his 
ponderings  upon  the  mind  of  Sir  Thomas  Dick-Lauder. 
His  solution  was  this:  Taking  all  their  features  into 
account,  he  was  convinced  that  water  only  could  have 
produced  the  terraces.  But  how  had  the  water  been 
collected?  He  saw  clearly  that,  supposing  the  mouth 
of  Glen  Gluoy  to  be  stopped  by  a  barrier  sufficiently 
high,  if  the  waters  from  the  mountains  flanking  the 
glen  were  allowed  to  collect,  they  would  form  behind 
the  barrier  a  lake,  the  surface  of  which  would  gradually 
rise  until  it  reached  the  level  of  the  col  at  the  head  of 
the  glen.  The  rising  would  then  cease;  the  superflu- 
ous water  of  Glen  Gluoy  discharging  itself  over  the  col 
into  Glen  Roy.  As  long  as  the  barrier  stopping  the 
mouth  of  Glen  Gluoy  continued  high  enough,  we 
should  have  in  that  glen  a  lake  at  the  precise  level  of 
its  shelf,  which  lake,  acting  upon  the  loose  drift  of  the 
flanking  mountains,  would  form  the  shelf  revealed  by 
observation. 

So  much  for  Glen  Gluoy.  But  suppose  the  mouth 
of  Glen  Roy  also  stopped  by  a  similar  barrier.  Behind 
it  also  the  water  from  the  adjacent  mountains  would 


210  FRAGMENTS    OF    SCIENCE. 

collect.  The  surface  of  the  lake  thus  formed  would 
gradually  rise,  until  it  had  reached  the  level  of  the 
col  which  divides  Glen  Eoy  from  Glen  Spey.  Here 
the  rising  of  the  lake  would  cease;  its  superabundant 
water  being  poured  over  the  col  into  the  valley  of  the 
Spey.  This  state  of  things  would  continue  as  long  as 
a  sufficiently  high  barrier  remained  at  the  mouth  of 
Glen  Eoy.  The  lake  thus  dammed  in,  with  its  surface 
at  the  level  of  the  highest  parallel  road,  would  act,  as 
in  Glen  Gluoy,  upon  the  friable  drift  overspreading  the 
mountains,  and  would  form  the  highest  road  or  terrace 
of  Glen  Eoy. 

And  now  let  us  suppose  the  barrier  to  be  so  far 
removed  from  the  mouth  of  Glen  Eoy  as  to  establish  a 
connection  between  it  and  the  upper  part  of  Glen 
Spean,  while  the  lower  part  of  the  latter  glen  still  con- 
tinued to  be  blocked  up.  Upper  Glen  Spean  and  Glen 
Eoy  would  then  be  occupied  by  a  continuous  lake,  the 
level  of  which  would  obviously  be  determined  by  the 
col  at  the  head  of  Loch  Laggan.  The  water  in  Glen 
Eoy  would  sink  from  the  level  it  had  previously  main- 
tained, to  the  level  of  its  new  place  of  escape.  This 
new  lake-surface  would  correspond  exactly  with  the 
lowest  parallel  road,  and  it  would  form  that  road  by  its 
action  upon  the  drift  of  the  adjacent  mountains. 

In  presence  of  the  observed  facts,  this  solution  com- 
mends itself  strongly  to  the  scientific  mind.  The 
question  next  occurs,  What  was  the  character  of  the 
assumed  barrier  which  stopped  the  glens?  There  are 
at  the  present  moment  vast  masses  of  detritus  in  cer- 
tain portions  of  Glen  Spean,  and  of  such  detritus  Sir 
Thomas  Dick-Lauder  imagined  his  barriers  to  have 
been  formed.  By  some  unknown  convulsion,  this  de- 
tritus had  been  heaped  up.  But,  once  given,  and  once 
granted  that  it  was  subsequently  removed  in  the  man- 


THE  PARALLEL  ROADS  OF  GLEN  ROY.  211 

ner  indicated,  the  single  road  of  Glen  Gluoy  and  the 
highest  and  lowest  roads  of  Glen  Roy  would  be  ex- 
plained in  a  satisfactory  manner. 

To  account  for  the  second  or  middle  road  of  Glen 
Roy,  Sir  Thomas  Dick-Lauder  invoked  a  new  agency. 
He  supposed  that  at  a  certain  point  in  the  breaking 
down  or  waste  of  his  dam,  a  halt  occurred,  the  barrier 
holding  its  ground  at  a  particular  level  sufficiently  long 
to  dam  a  lake  rising  to  the  height  of,  and  forming  the 
second  road.  This  point  of  weakness  was  at  once  de- 
tected by  Mr.  Darwin,  and  adduced  by  him  as  proving 
that  the  levels  of  the  cols  did  not  constitute  an  essen- 
tial feature  in  the  phenomena  of  the  parallel  roads. 
Though  not  destroyed,  Sir  Thomas  Dick-Lauder's  the- 
ory was  seriously  shaken  by  this  argument,  and  it  be- 
came a  point  of  capital  importance,  if  the  facts  per- 
mitted, to  remove  such  source  of  weakness.  This  was 
done  in  1847  by  Mr.  David  Milne,  now  Mr.  Milne- 
Home.  On  walking  up  Glen  Roy  from  Roy  Bridge, 
we  pass  the  mouth  of  a  lateral  glen,  called  Glen  Glas- 
ter,  running  eastward  from  Glen  Roy.  There  is  noth- 
ing in  this  lateral  glen  to  attract  attention,  or  to  sug- 
gest that  it  could  have  any  conspicuous  influence  in 
the  production  of  the  parallel  roads.  Hence,  probably, 
the  failure  of  Sir  Thomas  Dick-Lauder  to  notice  it. 
But  Mr.  Milne-Home  entered  this  glen,  on  the  north- 
ern side  of  which  the  middle  and  lowest  roads  are  fairly 
shown.  The  principal  stream  running  through  the 
glen  turns  at  a  certain  point  northwards  and  loses  itself 
among  hills  too  high  to  offer  any  outlet.  But  another 
branch  of  the  glen  turns  to  the  south-east;  and,  fol- 
lowing up  this  branch,  Mr.  Milne-Home  reached  a  col, 
or  water-shed,  of  the  precise  level  of  the  second  Glen 
Roy  road.  When  the  barrier  blocking  the  glens  had 
been  so  far  removed  as  to  open  this  col,  the  water  in 


212  FRAGMENTS    OF    SCIENCE. 

Glen  Roy  would  sink  to  the  level  of  the  second  road. 
A  new  lake  of  diminished  depth  would  be  thus  formed, 
the  surplus  water  of  which  would  escape  over  the  Glen 
Glaster  col  into  Glen  Spean.  The  margin  of  this  new 
lake,  acting  upon  the  detrital  matter,  would  form  the 
second  road.  The  theory  of  Sir  Thomas  Dick-Lauder, 
as  regards  the  part  played  by  the  cols,  was  re-riveted 
by  this  new  and  unexpected  discovery. 

I  have  referred  to  Mr.  Darwin,  whose  powerful 
mind  swayed  for  a  time  the  convictions  of  the  scientific 
world  in  relation  to  this  question.  His  notion  was — 
and  it  is  a  notion  which  very  naturally  presents  itself — 
that  the  parallel  roads  were  formed  by  the  sea;  that 
this  whole  region  was  once  submerged  and  subsequent- 
ly unheaved;  that  there  were  pauses  in  the  process  of 
upheaval,  during  which  these  glens  constituted  so 
many  fiords,  on  the  sides  of  which  the  parallel  ter- 
races were  formed.  This  theory  will  not  bear  close 
criticism;  nor  is  it  now  maintained  by  Mr.  Darwin  him- 
self. It  would  not  account  for  the  sea  being  20  feet 
higher  in  Glen  Gluoy  than  in  Glen  Eoy.  It  would  not 
account  for  the  absence  of  the  second  and  third  Glen 
Roy  roads  from  Glen  Gluoy,  where  the  mountain  flanks 
are  quite  as  impressionable  as  in  Glen  Roy.  It  would 
not  account  for  the  absence  of  the  shelves  from  the 
other  mountains  in  the  neighbourhood,  all  of  which 
would  have  been  clasped  by  the  sea  had  the  sea  been 
there.  Here  then,  and  no  doubt  elsewhere,  Mr.  Darwin 
has  shown  himself  to  be  fallible;  but  here,  as  elsewhere, 
he  has  shown  himself  equal  to  that  discipline  of  sur- 
render to  evidence  which  girds  his  intellect  with  such 
unassailable  moral  strength. 

But,  granting  the  significance  of  Sir  Thomas  Dick- 
Lauder's  facts,  and  the  reasonableness,  on  the  whole,  of 
the  views  which  he  has  founded  on  them,  they  will  not 


THE  PARALLEL  ROADS  OF  GLEX  ROY.   213 

bear  examination  in  detail.  No  such  barriers  of  de- 
tritus as  he  assumed  could  have  existed  without  leaving 
traces  behind  them;  but  there  is  no  trace  left.  There 
is  detritus  enough  in  Glen  Spean,  but  not  where  it  is 
wanted.  The  two  highest  parallel  roads  stop  abruptly 
at  different  points  near  the  mouth  of  Glen  Roy,  but 
no  remnant  of  the  barrier  against  which  they  abutted 
is  to  be  seen.  It  might  be  urged  that  the  subsequent 
invasion  of  the  valley  by  glaciers  has  swept  the  detritus 
away;  but  there  have  been  no  glaciers  in  these  valleys 
since  the  disappearance  of  the  lakes.  Professor  Geikie 
has  favoured  me  with  a  drawing  of  the  Glen  Spean 
'road'  near  the  entrance  to  Glen  Trieg.  The  road 
forms  a  shelf  round  a  great  mound  of  detritus  which, 
had  a  glacier  followed  the  formation  of  the  shelf,  must 
have  been  cleared  away.  Taking  all  the  circumstances 
into  account,  you  may,  I  think,  with  safety  dismiss  the 
detrital  barrier  as  incompetent  to  account  for  the  pres- 
ent condition  of  Glen  Gluoy  and  Glen  Roy. 

Hypotheses  in  science,  though  apparently  trans- 
cending experience,  are  in  reality  experience  modified 
by  scientific  thought  and  pushed  into  an  ultra-experi- 
ential region.  At  the  time  that  he  wrote,  Sir  Thomas 
Dick-Lauder  could  not  possibly  have  discerned  the 
cause  subsequently  assigned  for  the  blockage  of  these 
glens.  A  knowledge  of  the  action  of  ancient  glaciers 
was  the  necessary  antecedent  to  the  new  explanation, 
and  experience  of  this  nature  was  not  possessed  by  the 
distinguished  writer  just  mentioned.  The  extension  of 
Swiss  glaciers  far  beyond  their  present  limits,  was  first 
made  known  by  a  Swiss  engineer  named  Venetz,  who 
established,  by  the  marks  they  had  left  behind  them, 
their  former  existence  in  places  which  they  had  long 
forsaken.  The  subject  of  glacier  extension  was  subse- 
quently followed  up  with  distinguished  success  by 


214  FRAGMENTS    OF    SCIENCE. 

Charpentier,  Studer,  and  others.  With  characteristic 
vigour  Agassiz  grappled  with  it,  extending  his  observa- 
tions far  beyond  the  dpmain  of  Switzerland.  He  came 
to  this  country  in  1840,  and  found  in  various  places 
indubitable  marks  of  ancient  glacier  action.  England, 
Scotland,  Wales,  and  Ireland  he  proved  to  have  once 
given  birth  to  glaciers.  He  visited  Glen  Eoy,  sur- 
veyed the  surrounding  neighbourhood,  and  pronounced, 
as  a  consequence  of  his  investigation, the  barriers  which 
stopped  the  glens  and  produced  the  parallel  roads  to 
have  been  barriers  of  ice.  To  Mr.  Jamieson,  above  all 
others,  we  are  indebted  for  the  thorough  testing  and 
confirmation  of  this  theory. 

And  let  me  here  say  that  Agassiz  is  only  too  likely 
to  be  misrated  and  misjudged  by  those  who,  though 
accurate  within  a  limited  sphere,  fail  to  grasp  in  their 
totality  the  motive  powers  invoked  in  scientific  inves- 
tigation. True  he  lacked  mechanical  precision,  but  he 
abounded  in  that  force  and  freshness  of  the  scientific 
imagination  which  in  some  sciences,  and  probably  in 
some  stages  of  all  sciences,  are  essential  to  the  creator 
of  knowledge.  To  Agassiz  was  given,  not  the  art  of 
the  refiner,  but  the  instinct  of  the  discoverer,  and  the 
strength  of  the  delver  who  brings  ore  from  the  recesses 
of  the  mine.  That  ore  may  contain  its  share  of  dross, 
but  it  also  contains  the  precious  metal  which  gives 
employment  to  the  refiner,  and  without  which  his 
occupation  would  depart. 

Let  us  dwell  for  a  moment  upon  this  subject  of 
ancient  glaciers.  Under  a  flask  containing  water,  in 
which  a  thermometer  is  immersed,  is  placed  a  Bunsen's 
lamp.  The  water  is  heated,  reaches  a  temperature  of 
212°,  and  then  begins  to  boil.  The  rise  of  the  ther- 
mometer then  ceases,  although  heat  continues  to  be 
poured  by  the  lamp  into  the  water.  What  becomes  of 


THE    PARALLEL    ROADS    OF    GLEN    ROY.      215 

that  heat?  We  know  that  it  is  consumed  in  the  mo- 
lecular work  of  vaporization.  In  the  experiment  here 
arranged,  the  steam  passes  from  the  flask  through  a 
tube  into  a  second  vessel  kept  at  a  low  temperature. 
Here  it  is  condensed,  and  indeed  congealed  to  ice,  the 
second  vessel  being  plunged  in  a  mixture  cold  enough 
to  freeze  the  water.  As  a  result  of  the  process  we 
obtain  a  mass  of  ice.  That  ice  has  an  origin  very 
antithetical  to  its  own  character.  Though  cold,  it  is 
the  child  of  heat.  If  we  removed  the  lamp,  there 
would  be  no  steam,  and  if  there  were  no  steam  there 
would  be  no  ice.  The  mere  cold  of  the  mixture  sur- 
rounding the  second  vessel  would  not  produce  ice.  The 
cold  must  have  the  proper  material  to  work  upon;  and 
this  material — aqueous  vapour — is,  as  we  here  see,  the 
direct  product  of  heat. 

It  is  now,  I  suppose,  fifteen  or  sixteen  years  since  I 
found  myself  conversing  with  an  illustrious  philoso- 
pher regarding  that  glacial  epoch  which  the  researches 
of  Agassiz  and  others  had  revealed.  This  profoundly 
thoughtful  man  maintained  the  fixed  opinion  that,  at  a 
certain  stage  in  the  history  of  the  solar  system,  the 
sun's  radiation  had  suffered  diminution,  the  glacial 
epoch  being  a  consequence  of  this  solar  chill.  The 
celebrated  French  mathematician  -Poisson  had  another 
theory.  Astronomers  have  shown  that  the  solar  system 
moves  through  space,  and  '  the  temperature  of  space ' 
is  a  familiar  expression  with  scientific  men.  It  was 
considered  probable  by  Poisson  that  our  system,  dur- 
ing its  motion,  had  traversed  portions  of  space  of  dif- 
ferent temperatures;  and  that,  during  its  passage 
through  one  of  the  colder  regions  of  the  universe,  the 
glacial  epoch  occurred.  Notions  such  as  these  were 
more  or  less  current  everywhere  not  many  years  ago, 
and  I  therefore  thought  it  worth  while  to  show  how 

15 


216  FRAGMENTS    OF    SCIENCE. 

incomplete  they  were.  Suppose  the  temperature  of  our 
planet  to  be  reduced,  by  the  subsidence  of  solar  heat, 
the  cold  of  space,  or  any  other  cause,  say  one  hundred 
degrees.  Four-and-twenty  hours  of  such  a  chill  would 
bring  down  as  snow  nearly  all  the  moisture  of  our  at- 
mosphere. But  this  would  not  produce  a  glacial  epoch. 
Such  an  epoch  would  require  the  long-continued  gen- 
eration of  the  material  from  which  the  ice  of  glaciers 
is  derived.  Mountain  snow,  the  nutriment  of  glaciers, 
is  derived  from  aqueous  vapour  raised  mainly  from  the 
tropical  ocean  by  the  sun.  The  solar  fire  is  as  neces- 
sary a  factor  in  the  process  as  our  lamp  in  the  experi- 
ment referred  to  a  moment  ago.  Nothing  is  easier 
than  to  calculate  the  exact  amount  of  heat  expended 
by  the  sun  in  the  production  of  a  glacier.  It  would, 
as  I  have  elsewhere  shown,*  raise  a  quantity  of  cast 
iron  five  times  the  weight  of  the  glacier  not  only  to  a 
white  heat,  but  to  its  point  of  fusion.  If,  as  I  have 
already  urged,  instead  of  being  filled  with  ice,  the 
valleys  of  the  Alps  were  filled  with  white-hot  metal,  of 
quintuple  the  mass  of  the  present  glaciers,  it  is  the 
heat,  and  not  the  cold,  that  would  arrest  our  attention 
and  solicit  our  explanation.  The  process  of  glacier 
making  is  obviously  one  of  distillation,  in  which  the 
fire  of  the  sun,  which  generates  the  vapour,  plays  as 
essential  a  part  as  the  cold  of  the  mountains  which 
condenses  it.f 

It  was  their  ascription  to  glacier  action  that  first 

*  '  Heat  a  Mode  of  Motion,'  fifth  edition,  chap.  vi. :  Forms  of 
Water.  £g  55  and  56. 

f-  In  Lyell's  excellent  'Principles  of  Geology,'  the  remark  oc- 
curs that  'several  writers  have  fallen  into  the  strange  error  of 
supposing  that  the  glacial  period  must  have  been  one  of  higher 
mean  temperature  than  usual.'  The  really  strange  error  was  the 
forgetfulness  of  the  fact  that  without  the  heat  the  substance 
necessary  to  the  production  of  glaciers  would  be  wanting. 


THE    PARALLEL    ROADS    OF    GLEN    ROY.      217 

gave  the  parallel  roads  of  Glen  Roy  an  interest  in  my 
eyes;  and  in  1867,  with  a  view  to  self-instruction,  I 
made  a  solitary  pilgrimage  to  the  place,  and  explored 
pretty  thoroughly  the  roads  of  the  principal  glen.  I 
traced  the  highest  road  to  the  col  dividing  Glen  Roy 
from  Glen  Spey,  and,  thanks  to  the  civility  of  an 
Ordnance  surveyor,  I  was  enabled  to  inspect  some  of 
the  roads  with  a  theodolite,  and  to  satisfy  myself  re- 
garding the  common  level  of  the  shelves  at  opposite 
sides  of  the  valley.  As  stated  by  Pennant,  the  width 
of  the  roads  amounts  sometimes  to  more  than  twenty 
yards;  but  near  the  head  of  Glen  Roy  the  highest  road 
ceases  to  have  any  width,  for  it  runs  along  the  face  of 
a  rock,  the  effect  of  the  lapping  of  the  water  on  the 
more  friable  portions  of  the  rock  being  perfectly  dis- 
tinct to  this  hour.  My  knowledge  of  the  region  was, 
however,  far  from  complete,  and  nine  years  had 
dimmed  the  memory  even  of  the  portion  which  had 
been  thoroughly  examined.  Hence  my  desire  to  see 
the  roads  once  more  before  venturing  to  talk  to  you 
about  them.  The  Easter  holidays  of  1876  were  to  be 
devoted  to  this  purpose;  but  at  the  last  moment  a  tele- 
gram from  Roy  Bridge  informed  me  that  the  roads 
were  snowed  up.  Finding  books  and  memories  poor 
substitutes  for  the  flavour  of  facts,  I  resolved  subse- 
quently to  make  another  effort  to  see  the  roads.  Ac- 
cordingly last  Thursday  fortnight,  after  lecturing  here, 
I  packed  up,  and  started  (not  this  time  alone)  for  the 
North.  Next  day  at  noon  my  wife  and  I  found  our- 
selves at  Dalwhinnio,  whence  a  drive  of  some  five-and- 
thirty  miles  brought  us  to  the  excellent  hostelry  of 
Mr.  Macintosh,  at  the  mouth  of  Glen  Roy. 

We  might  have  found  the  hills  covered  with  mist, 
which  would  have  wholly  defeated  us;  but  Nature  was 
good-natured,  and  we  had  two  successful  working  days 


218  FRAGMENTS    OF    SCIENCE. 

among  the  hills.  Guided  by  the  excellent  ordnance 
map  of  the  region,  on  the  Saturday  morning  we  went 
up  the  glen,  and  on  reaching  the  stream  called  Allt 
Bhreac  Achaidh  faced  the  hills  to  the  west.  At  the 
watershed  between  Glen  Roy  and  Glen  Fintaig  we  bore 
northwards,  struck  the  ridge  above  Glen  Gluoy,  came 
in  view  of  its  road,  which  we  persistently  followed  as 
long  as  it  continued  visible.  It  is  a  feature  of  all  the 
roads  that  they  vanish  before  reaching  the  cols  over 
which  fell  the  waters  of  the  lakes  which  formed  them. 
One  reason  doubtless  is  that  at  their  upper  ends  the 
lakes  were  shallow,  and  incompetent  on  this  account 
to  raise  wavelets  of  any  strength  to  act  upon  the  moun- 
tain drift.  A  second  reason  is  that  they  were  land- 
locked in  the  higher  portions  and  protected  from  the 
south-westerly  winds,  the  stillness  of  their  waters  caus- 
ing them  to  produce  but  a  feeble  impression  upon  the 
mountain  sides.  From  Glen  Gluoy  we  passed  down 
Glen  Turrit  to  Glen  Roy,  and  through  it  homewards, 
thus  accomplishing  two  or  three  and  twenty  miles  of 
rough  and  honest  work. 

Next  day  we  thoroughly  explored  Glen  Glaster, 
following  its  two  roads  as  far  as  they  were  visible.  We 
reached  the  col  discovered  by  Mr.  Milne-Home,  which 
stands  at  the  level  of  the  middle  road  of  Glen  Roy. 
Thence  we  crossed  southwards  over  the  mountain  Creag 
DhuWi,  and  examined  the  erratic  blocks  upon  its  sides, 
and  the  ridges  and  mounds  of  moraine  matter  which 
cumber  the  lower  flanks  of  the  mountain.  The  ob- 
servations of  Mr.  Jamieson  upon  this  region,  including 
the  mouth  of  Glen  Trieg,  are  in  the  highest  degree 
interesting.  We  entered  Glen  Spean,  and  continued 
a  search  begun  on  the  evening  of  our  arrival  at  Roy 
Bridge — the  search,  namely,  for  glacier  polishings  and 
markings.  We  did  not  find  them  copious,  but  they  are 


THE    PARALLEL    ROADS    OF    GLEN    ROY.      219 

indubitable.  One  of  the  proofs  most  convenient  for 
reference,  is  a  great  rounded  rock  by  the  roadside, 
1,000  yards  east  of  the  milestone  marked  three-quarters 
of  a  mile  from  Roy  Bridge.  Farther  east  other  cases 
occur,  and  they  leave  no  doubt  upon  the  mind  that 
Glen  Speanwas  at  one  time  filled  by  a  great  glacier.  To 
the  disciplined  eye  the  aspect  of  the  mountains  is  per- 
fectly conclusive  on  this  point;  and  in  no  position  can 
the  observer  more  readily  and  thoroughly  convince 
himself  of  this  than  at  the  head  of  Glen  Glaster.  The 
dominant  hills  here  are  all  intensely  glaciated. 

But  the  great  collecting  ground  of  the  glaciers 
which  dammed  the  glens  and  produced  the  parallel 
roads,  were  the  mountains  south  and  west  of  Glen 
Spean.  The  monarch  of  these  is  Ben  Nevis,  4,370  feet 
high.  The  position  of  Ben  Nevis  and  his  colleagues,  in 
reference  to  the  vapour-laden  winds  of  the  Atlantic,  is 
a  point  of  the  first  importance.  It  is  exactly  similar  to 
that  of  Carrantual  and  the  Macgillicuddy  Reeks  in  the 
south-west  of  Ireland.  These  mountains  are,  and  were, 
the  first  to  encounter  the  south-western  Atlantic  winds, 
and  the  precipitation,  even  at  present,  in  the  neigh- 
bourhood of  Killaraey,  is  enormous.  The  winds, 
robbed  of  their  vapour,  and  charged  with  the  heat  set 
free  by  its  precipitation,  pursue  their  direction  oblique- 
ly across  Ireland;  and  the  effect  of  the  drying  process 
may  be  understood  by  comparing  the  rainfall  at  Ca- 
ll irciveen  with  that  at  Portarlington.  As  found  by 
Dr.  Lloyd,  the  ratio  is  as  59  to  21 — fifty-nine  inches 
annually  at  Cahirciveen  to  twenty-one  at  Portarling- 
ton. During  the  glacial  epoch  this  vapour  fell  as  snow, 
and  the  consequence  was  a  system  of  glaciers  which 
have  left  traces  and  evidences  of  the  most  impressive 
character  in  the  region  of  the  Killarney  Lakes.  I  have 
referred  in  other  places  to  the  great  glacier  which, 


220  FEAGMENTS    OF    SCIENCE. 

descending  from  the  Beeks,  moved  through  the  Black 
Valley,  took  possession  of  the  lake-basins,  and  left  its 
traces  on  every  rock  and  island  emergent  from  the 
waters  of  the  upper  lake.  They  are  all  conspicuously 
glaciated.  Not  in  Switzerland  itself  do  we  find  clearer 
traces  of  ancient  glacier  action. 

What  the  Macgillicuddy  Eeeks  did  in  Ireland,  Ben 
Nevis  and  the  adjacent  mountains  did,  and  continue  to 
do,  in  Scotland.  We  had  an  example  of  this  on  the 
morning  we  quitted  Koy  Bridge.  From  the  bridge 
westward  rain  fell  copiously,  and  the  roads  were  wet; 
but  the  precipitation  ceased  near  Loch  Laggan,  whence 
eastward  the  roads  were  dry.  Measured  by  the  gauge, 
the  rainfall  at  Fort  William  is  86  inches,  while  at 
Laggan  it  is  only  46  inches  annually.  The  difference 
between  west  and  east  is  forcibly  brought  out  by  ob- 
servations at  the  two  ends  of  the  Caledonian  Canal. 
Fort  William  at  the  south-western  end  has,  as  just 
stated,  86  inches,  while  Culloden,  at  its  north-eastern 
end,  has  only  24.  To  the  researches  of  that  able  and 
accomplished  meteorologist,  Mr.  Buchan,  wre  are  in- 
debted for  these  and  other  data  of  the  most  interesting 
and  valuable  kind. 

Adhering  to  the  facts  now  presented  to  us,  it  is  not 
difficult  to  restore  in  idea  the  process  by  which  the 
glaciers  of  Lochaber  were  produced  and  the  glens 
dammed  by  ice.  When  the  cold  of  the  glacial  epoch 
began  to  invade  the  Scottish  hills,  the  sun  at  the  same 
time  acting  with  sufficient  power  upon  the  tropical 
ocean,  the  vapours  raised  and  drifted  on  to  these  north- 
ern mountains  were  more  and  more  converted  into 
snow.  This  slid  down  the  slopes,  and  from  every  val- 
ley, strath,  and  corry,  south  of  Glen  Spean,  glaciers 
were  poured  into  that  glen.  The  two  great  factors 
here  brought  into  play  are  the  nutrition  of  the  glaciers 


THE  PARALLEL  ROADS  OF  GLEN  ROY.   221 

by  the  frozen  material  above,  and  their  consumption  in 
the  milder  air  below.  For  a  period  supply  exceeded 
consumption,  and  the  ice  extended,  filling  Glen  Spean 
to  an  ever-increasing  height,  and  abutting  against  the 
mountains  to  the  north  of  that  glen.  But  why,  it  may 
be  asked,  should  the  valleys  south  of  Glen  Spean  be 
receptacles  of  ice  at  a  time  when  those  north  of  it  were 
receptacles  of  water?  The  answer  is  to  be  found  in 
the  position  and  the  greater  elevation  of  the  mountains 
south  of  Glen  Spean.  They  first  received  the  loads  of 
moisture  carried  by  the  Atlantic  winds,  and  not  until 
they  had  been  in  part  dried,  and  also  warmed  by  the 
liberation  of  their  latent  heat,  did  these  winds  touch 
the  hills  north  of  the  Glen. 

An  instructive  observation  bearing  upon  this  point 
is  here  to  be  noted.  Had  our  visit  been  in  the  winter 
we  should  have  found  all  the  mountains  covered;  had 
it  been  in  the  summer  we  should  have  found  the  snow 
all  gone.  But  happily  it  was  at  a  season  when  the 
aspect  of  the  mountains  north  and  south  of  Glen  Spean 
exhibited  their  relative  powers  as  snow  collectors. 
Scanning  the  former  hills  from  many  points  of  view, 
we  were  hardly  able  to  detect  a  fleck  of  snow,  while 
heavy  swaths  and  patches  loaded  the  latter.  Were  the 
glacial  epoch  to  return,  the  relation  indicated  by  this 
observation  would  cause  Glen  Spean  to  be  filled  with 
glaciers  from  the  south,  while  the  hills  and  valleys  on 
the  north,  visited  by  warmer  and  drier  winds,  would 
remain  comparatively  free  from  ice.  This  flow  from 
the  south  would  be  reinforced  from  the  west,  and  as 
long  as  the  supply  was  in  excess  of  the  consumption 
the  glaciers  would  extend,  the  dams  which  closed  the 
glens  increasing  in  height.  By-and-by  supply  and  con- 
sumption becoming  approximately  equal,  the  height  of 
the  glacier  barriers  would  remain  constant.  Then,  if 


222  FRAGMENTS    OF    SCIENCE. 

milder  weather  set  in,  consumption  would  be  in  excess, 
a  lowering  of  the  barriers  and  a  retreat  of  the  ice  being 
the  consequence.  But  for  a  long  time  the  conflict 
between  supply  and  consumption  would  continue,  re- 
tarding indefinitely  the  disappearance  of  the  barriers, 
and  keeping  the  imprisoned  lakes  in  the  northern 
glens.  But  however  slow  its  retreat,  the  ice  in  the 
long  run  would  be  forced  to  yield.  The  dam  at  the 
mouth  of  Glen  Boy,  which  probably  entered  the  glen 
sufficiently  far  to  block  up  Glen  Glaster,  would  grad- 
ually retreat.  Glen  Glaster  and  its  col  being  opened, 
the  subsidence  of  the  lake  eighty  feet,  from  the  level 
of  the  highest  to  that  of  the  second  parallel  road, 
would  follow  as  a  consequence.  I  think  this  the  most 
probable  course  of  things,  but  it  is  also  possible  that 
Glen  Glaster  may  have  been  blocked  by  a  glacier  from 
Glen  Trieg.  The  ice  dam  continuing  to  retreat,  at 
length  permitted  Glen  Eoy  to  connect  itself  with 
upper  Glen  Spean.  A  continuous  lake  then  filled  both 
glens,  the  level  of  which,  as  already  explained,  was 
determined  by  the  col  at  Makul,  above  the  head  of 
Loch  Laggan.  The  last  to  yield  was  the  portion  of  the 
glacier  which  derived  nutrition  from  Ben  Nevis,  and 
probably  also  from  the  mountains  north  and  south  of 
Loch  Arkaig.  But  it  at  length  yielded,  and  the  waters 
in  the  glens  resumed  the  courses  which  they  pursue 
to-day. 

For  the  removal  of  the  ice  barriers  no  cataclysm  is 
to  be  invoked;  the  gradual  melting  of  the  dam  would 
produce  the  entire  series  of  phenomena.  In  sinking 
from  col  to  col  the  water  would  flow  over  a  gradually 
melting  barrier,  the  surface  of  the  imprisoned  lake  not 
remaining  sufficiently  long  at  any  particular  level  to 
produce  a  shelf  comparable  to  the  parallel  roads.  By 
temporary  halts  in  the  process  of  melting  due  to  at- 


THE    PARALLEL    ROADS    OF    GLEN    ROY.      223 

mospheric  conditions  or  to  the  character  of  the  dam 
itself,  or  through  local  softness  in  the  drift,  small 
pseudo-terraces  would  be  formed,  which,  to  the  per- 
plexity of  some  observers,  are  seen  upon  the  flanks  of 
the  glens  to-day. 

In  presence  then  of  the  fact  that  the  barriers  which 
stopped  these  glens  to  a  height,  it  may  be,  of  1,500 
feet  above  the  bottom  of  Glen  Spean,  have  dissolved 
and  left  not  a  wreck  behind;  in  presence  of  the  fact, 
insisted  on  by  Professor  Geikie,  that  barriers  of  detri- 
tus would  undoubtedly  have  been  able  to  maintain 
themselves  had  they  ever  been  there;  in  presence  of 
the  fact  that  great  glaciers  once  most  certainly  filled 
these  valleys — that  the  whole  region,  as  proved  by  Mr. 
Jamieson,  is  filled  with  the  traces  of  their  action;  the 
theory  which  ascribes  the  parallel  roads  to  lakes 
dammed  by  barriers  of  ice  has,  in  my  opinion,  a  degree 
of  probability  on  its  side  which  amounts  to  a  practical 
demonstration  of  its  truth. 

Into  the  details  of  the  terrace  formation  I  do  not 
enter.  Mr.  Darwin  and  Mr.  Jamieson  on  the  one  side, 
and  Sir  John  Lubbock  on  the  other,  deal  with  true 
causes.  The  terraces,  no  doubt,  are  due  in  part  to  the 
descending  drift  arrested  by  the  water,  and  in  part  to 
the  fretting  of  the  wavelets,  and  the  rearrangement  of 
the  stirred  detritus,  along  the  belts  of  contact  of  lake 
and  hill.  The  descent  of  matter  must  have  been  fre- 
quent when  the  drift  was  unbound  by  the  rootlets 
which  hold  it  together  now.  In  some  cases,  it  may  be 
remarked,  the  visibility  of  the  roads  is  materially  aug- 
mented by  differences  of  vegetation.  The  grass  upon 
the  terraces  is  not  always  of  the  same  character  as  that 
nbove  and  below  them,  while  on  heather-covered  hills 


224  FKAGMENTS    OF    SCIENCE. 

The  annexed  sketch  of  a  model  (p.  225)  will  enable 
the  reader  to  grasp  the  essential  features  of  the  prob- 
lem and  its  solution.  Glen  Gluoy  and  Glen  Eoy  are 
lateral  valleys  which  open  into  Glen  Spean.  Let  us 
suppose  Glen  Spean  filled  from  v  to  w  with  ice  of  a  uni- 
form elevation  of  1,500  feet  above  the  sea,  the  ice  not 
filling  the  upper  part  of  that  glen.  The  ice  would 
thrust  itself  for  some  distance  up  the  lateral  valleys, 
closing  all  their  mouths.  The  streams  from  the  moun- 
tains right  and  left  of  Glen  Gluoy  would  pour  their 
waters  into  that  glen,  forming  a  lake,  the  level  of 
which  would  be  determined  by  the  height  of  the  col  at 

A,  1,170  feet  above  the  sea.     Over  this  col  the  water 
would  flow  into  Glen  Eoy.    But  in  Glen  Eoy  it  could 
not  rise  higher  than  1,150  feet,  the  height  of  the  col  at 

B,  over  which  it  would  flow  into  Glen  Spey. 

The  water  halting  at  these  levels  for  a  sufficient 
time,  would  form  the  single  road  in  Glen  Gluoy  and  the 
highest  road  in  Glen  Eoy.  This  state  of  things  would 
continue  as  long  as  the  ice  dam  was  sufficiently  high 
to  dominate  the  cols  at  A  and  B;  but  when  through 
change  of  climate  the  gradually  sinking  dam  reached, 
in  succession,  the  levels  of  these  cols,  the  water  would 
then  begin  to  flow  over  the  dam  instead  of  over  the 
cols.  Let  us  suppose  the  wasting  of  the  ice  to  con- 
tinue until  a  connection  was  established  between  Glen 
Eoy  and  Glen  Glaster,  a  common  lake  would  then  fill 
both  these  glens,  the  level  of  which  would  be  deter- 
mined by  that  of  the  col  c,  over  which  the  water  would 
pour  for  an  indefinite  period  into  Glen  Spean.  Dur- 
ing this  period  the  second  Glen  Eoy  road  and  the  high- 
est road  of  Glen  Glaster  would  be  formed.  The  ice 
subsiding  still  further,  a  connection  would  eventually 
be  established  between  Glen  Eoy,  Glen  Glaster,  and 
the  upper  part  of  Glen  Spean.  A  common  lake  would 


THE   PARALLEL    ROADS    OF    GLEN    ROY.     225 


226  FRAGMENTS    OF    SCIENCE. 

fill  all  three  glens,  the  level  of  which  would  be  that  of 
the  col  D,  over  which  for  an  indefinite  period  the  lake 
would  pour  its  water.  During  this  period  the  lowest 
Glen  Eoy  road,  which  is  common  also  to  Glen  Glaster 
and  Glen  Spean,  would  be  formed.  Finally,  on  the  dis- 
appearance of  the  ice  from  the  lower  part  of  Glen 
Spean  the  waters  would  flow  down  their  respective  val- 
leys as  they  do  to-day. 

Eeviewing  our  work,  we  find  three  considerable 
steps  to  have  marked  the  solution  of  the  problem  of 
the  Parallel  Eoads  of  Glen  Eoy.  The  first  of  these 
was  taken  by  Sir  Thomas  Dick-Lauder,  the  second  was 
the  pregnant  conception  of  Agassiz  regarding  glacier 
action,  and  the  third  was  the  testing  and  verification 
of  this  conception  by  the  very  thorough  researches  of 
Mr.  Jamieson.  No  circumstance  or  incident  connected 
with  this  discourse  gives  me  greater  pleasure  than  the 
recognition  of  the  value  of  these  researches.  They  are 
marked  throughout  by  unflagging  industry,  by  novelty 
and  acuteness  of  observation,  and  by  reasoning  power 
of  a  high  and  varied  kind.  These  pages  had  been  re- 
turned ( for  press '  when  I  learned  that  the  relation  of 
Ben  Nevis  and  his  colleagues  to  the  vapour-laden 
winds  of  the  Atlantic  had  not  escaped  Mr.  Jamieson. 
To  him  obviously  the  exploration  of  Lochaber,  and  the 
development  of  the  theory  of  the  Parallel  Eoads,  has 
been  a  labour  of  love. 

Thus  ends  our  rapid  survey  of  this  brief  episode 
in  the  physical  history  of  the  Scottish  hills, — brief, 
that  is  to  say,  in  comparison  with  the  immeasurable 
lapses  of  time  through  which,  to  produce  its  varied 
structure  and  appearances,  our  planet  must  have 
passed.  In  the  survey  of  such  a  field  two  things  are 
specially  worthy  to  be  taken  into  account — the  widen- 
ing of  the  intellectual  horizon  and  the  reaction  of  ex- 


4 1 

r 


m 


THE    PARALLEL    ROADS    OF    GLEX    ROY.      227 

panding  knowledge  upon  the  intellectual  organ  itself. 
At  first,  as  in  the  case  of  ancient  glaciers,  through 
sheer  want  of  capacity,  the  mind  refuses  to  take  in 
revealed  facts.  But  by  degrees  the  steady  contempt 
tion  of  these  facts  so  strengthens  and  expands  the  in- 
tellectual powers,  that  where  truth  once  could  not  find 
an  entrance  it  eventually  finds  a  home.* 

A  map  of  the  district,  with  the  parallel  roads  shown 
in  red,  is  annexed. 


LITERATURE  OF  THE  SUBJECT. 

THOMAS  PENNANT.— A  Tour  in  Scotland.     Vol.  iii.  1776,  p.  394. 
JOHN  MAcCuLLOCH. — On  the  Parallel  Roads  of  Glen  Roy.    Geol. 

Soc.  Trans,  vol.  iv.  1817,  p.  314. 
THOMAS  LAUDEB  DICK  (afterwards  SIR  THOMAS  DICK-LAUDEK, 

Bart.)— On  the  Parallel  Roads  of  Lochaber.    Edin.  Roy.  Soc. 

Trans.  1818,  vol.  is.  p.  1. 
CHARLES  DARWIN.— Observations  on  the  Parallel  Roads  of  Glen 

Roy,  and  of  the  other  parts  of  Lochaber  in  Scotland,  with  an 

attempt  to  prove  that  they  are  of  marine  origin.     Phil. 

Trans.  1839,  vol.  cxxix.  p.  39. 
SIR  CHARLES  LYELL. — Elements  of  Geology.    Second  edition, 

1841. 


*  The  formation,  connection,  successive  subsidence,  and  final 
disappearance  of  the  glacial  lakes  of  Lochaber  were  illustrated  in 
the  discourse  here  reported  by  the  model  just  described,  con- 
structed under  the  supervision  of  my  assistant,  Mr.  John  Cottrell. 
(Men  Gluoy  with  its  lake  and  road  and  the  cataract  over  its  col; 
Glen  Roy  and  its  three  roads  with  their  respective  cataracts  at 
the  head  of  Glon  Spey,  Glen  Glaster,  and  Glen  Spean,  were  nil 
represented.  The  successive  shiftings  of  the  barriers,  which 
were  formed  of  plate  glass,  brought  each  successive  lake  and  its 
corresponding  road  into  view,  while  the  entire  removal  of  the 
barriers  caused  the  streams  to  flow  down  the  glens  of  the  model 
as  they  flow  down  the  real  glens  of  to-day. 


228  FRAGMENTS    OF    SCIENCE. 

Louis  AOASSIZ. — The  Glacial  Theory  and  its  Recent  Progress — 

Parallel    Terraces.      Edin.  New   Phil.  Journal,   1842,   vol. 

xxxiii.  p.  236. 
DAVID  MILNE  (afterward  DAVID  MILNE-HOME).— On  the  Parallel 

Eoads  of  Lochaber;  with  Remarks  on  the  Change  of  Rela- 
tive Levels  of  Sea  and  Land  in  Scotland,  and  on  the  Detrit;il 

Deposits  in  that  Country.    Edin.  Roy.  Soc.  Trans.  1847,  vol. 

xvi.  p.  395. 

ROBERT  CHAMBERS. — Ancient  Sea  Margins.     Edinburgh,  1848. 
H.  D.  ROGERS.— On  the  Parallel  Roads  of  Glen  Roy.    Royal  Inst. 

Proceedings,  1861,  vol.  iii.  p.  341. 
THOMAS  F.  JAMIESON.— On  the  Parallel  Roads  of  Glen  Roy,  and 

their  Place  in  the  History  of  the  Glacial  Period.    Quart. 

Journal  Geol.  Soc.  1863,  vol.  xix.  p.  235. 
SIR  CHARLES  LYELL. — Antiquity  of  Man.     1863,  p.  253. 
REV.  R.  B.  WATSON.— On  the  Marine  Origin  of  the  Parallel 

Roads  of  Glen  Roy.     Quart.  Journ.  Geol.  Soc.  1865,  vol. 

xxii.  p.  9. 
SIR  JOHN  LUBBOCK.— On  the  Parallel  Roads  of  Glen  Roy.    Quart. 

Journ.  Geol.  Soc.  1867,  vol.  xxiv.  p.  83. 
CHARLES  BABBAGE. — Observations  on  the  Parallel  Roads  of  Glen 

Roy.    Quart.  Journ.  Geol.  Soc.  1868,  vol.  xxiv.  p.  273. 
JAMES  NICOL. — On  the  Origin  of  the  Parallel  Roads  of  Glen  Roy. 

1869.    Geol.  Soc.  Journal,  vol  xxv.  p.  282. 
JAMES  NICOL. — How  the  Parallel  Roads  of  Glen  Roy  were  formed. 

1872.     Geol.  Soc.  Journal,  vol.  xxviii.  p.  237. 
MAJOR-GENERAL  SIR  HENRY  JAMES,  R.  E.— Notes  on  the  Parallel 

Roads  of  Lochaber.    4to.    1874. 


IX. 

ALPINE  SCULPTURE. 
18G4. 

TO  account  for  the  conformation  of  the  Alps,  two 
hypotheses  have  been  advanced,  which  may  be 
respectively  named  the  hypothesis  of  fracture  and  the 
hypothesis  of  erosion.  The  former  assumes  that  the 
forces  by  which  the  mountains  were  elevated  produced 
fissures  in  the  earth's  crust,  and  that  the  valleys  of  the 
Alps  are  the  tracks  of  these  fissures;  while  the  latter 
maintains  that  the  valleys  have  been  cut  out  by  the 
action  of  ice  and  water,  the  mountains  themselves 
being  the  residual  forms  of  this  grand  sculpture.  I 
had  heard  the  Via  Mala  cited  as  a  conspicuous  illus- 
tration of  the  fissure  theory — the  profound  chasm  thus 
named,  and  through  which  the  Hinter-Ehein  now 
flows,  could,  it  was  alleged,  be  nothing  else  than  a  crack 
in  the  earth's  crust.  To  the  Via  Mala  I  therefore  went 
in  1864  to  instruct  myself  upon  the  point  in  question. 
The  gorge  commences  about  a  quarter  of  an  hour 
above  Tusis;  and,  on  entering  it,  the  first  impression 
certainly  is  that  it  must  be  a  fissure.  This  conclusion 
in  my  case  was  modified  as  I  advanced.  Some  distance 
up  the  gorge  I  found  upon  the  slopes  to  my  right 
quantities  of  rolled  stones,  evidently  rounded  by  water- 
action.  Still  further  up,  and  just  before  reaching  the 
first  bridge  which  spans  the  chasm,  I  found  more  rolled 
stones,  associated  with  sand  and  gravel.  Through  this 
of  detritus,  fortunately,  a  vertical  cutting  had 


230  FRAGMENTS    OF    SCIENCE. 

been  made,  which  exhibited  a  section  showing  perfect 
stratification.  There  was  no  agency  in  the  place  to  roll 
these  stones,  and  to  deposit  these  alternating  layers  of 
sand  and  pebbles,  but  the  river  which  now  rushes  some 
hundreds  of  feet  below  them.  At  one  period  of  the  Via 
Mala's  history  the  river  must  have  run  at  this  high 
level.  Other  evidences  of  water-action  soon  revealed 
themselves.  From  the  parapet  of  the  first  bridge  I 
could  see  the  solid  rock  200  feet  above  the  bed  of  the 
river  scooped  and  eroded. 

It  is  stated  in  the  guide-books  that  the  river,  which 
usually  runs  along  the  bottom  of  the  gorge,  has  been 
known  almost  to  fill  it  during  violent  thunder-storms; 
and  it  may  be  urged  that  the  marks  of  erosion  which 
the  sides  of  the  chasm  exhibit  are  due  to  those  occa- 
sional floods.  In  reply  to  this,  it  may  be  stated  that 
even  the  existence  of  such  floods  is  not  well  authenti- 
cated, and  that  if  the  supposition  were  true,  it  would 
be  an  additional  argument  in  favour  of  the  cutting 
power  of  the  river.  For  if  floods  operating  at  rare  inter- 
vals could  thus  erode  the  rock,  the  same  agency,  acting 
without  ceasing  upon  the  river's  bed,  must  certainly 
be  competent  to  excavate  it. 

I  proceeded  upwards,  and  from  a  point  near  an- 
other bridge  (which  of  them  I  did  not  note)  had  a  fine 
view  of  a  portion  of  the  gorge.  The  river  here  runs 
at  the  bottom  of  a  cleft  of  profound  depth,  but  so  nar- 
row that  it  might  be  leaped  across.  That  this  cleft 
must  be  a  crack  is  the  impression  first  produced;  but 
a  brief  inspection  suffices  to  prove  that  it  has  been  cut 
by  the  river.  From  top  to  bottom  we  have  the  unmis- 
takable marks  of  erosion.  This  cleft  was  best  seen  on 
looking  downwards  from  a  point  near  the  bridge;  but 
looking  upwards  from  the  bridge  itself,  the  evidence 
of  aqueous  erosion  was  equally  convincing. 


ALPINE    SCULPTURE.  231 

The  character  of  the  erosion  depends  upon  the  rock 
as  well  as  upon  the  river.  The  action  of  water  upon 
some  rocks  is  almost  purely  mechanical;  they  are  sim- 
ply ground  away  or  detached  in  sensible  masses. 
Water,  however,  in  passing  over  limestone,  charges  it- 
self with  carbonate  of  lime  without  damage  to  its  trans- 
parency; the  rock  is  dissolved  in  the  water;  and  the 
gorges  cut  by  water  in  such  rocks  often  resemble  those 
cut  in  the  ice  of  glaciers  by  glacier  streams.  To  the 
solubility  of  limestone  is  probably  to  be  ascribed  the 
fantastic  forms  which  peaks  of  this  rock  usually  as- 
sume, and  also  the  grottos  and  caverns  which  inter- 
penetrate limestone  formations.  A  rock  capable  of 
being  thus  dissolved  will  expose  a  smooth  surface  after 
the  water  has  quitted  it;  and  in  the  case  of  the  Via 
Mala  it  is  the  polish  of  the  surfaces  and  the  curved  hol- 
lows scooped  in  the  sides  of  the  gorge,  which  assure  us 
that  the  chasm  has  been  the  work  of  the  river. 

About  four  miles  from  Tusis,  and  not  far  from  the 
little  village  of  Zillis,  the  Via  Mala  opens  into  a  plain 
bounded  by  high  terraces.  It  occurred  to  me  the 
moment  I  saw  it  that  the  plain  had  been  the  bed  of  an 
ancient  lake;  and  a  farmer,  who  was  my  temporary 
companion,  immediately  informed  me  that  such  was 
the  tradition  of  the  neighbourhood.  This  man  con- 
versed with  intelligence,  and  as  I  drew  his  attention  to 
the  rolled  stones,  which  rest  not  only  above  the  river, 
but  above  the  road,  and  inferred  that  the  river  must 
once  have  been  there  to  have  rolled  those  stones,  he 
saw  the  force  of  the  evidence  perfectly.  In  fact,  in 
former  times,  and  subsequent  to  the  retreat  of  the 
great  glaciers,  a  rocky  barrier  crossed  the  valley  at  this 
place,  damming  the  river  which  came  from  the  moun- 
tains higher  up.  A  lake  was  thus  formed  which  poured 
its  waters  over  the  barrier.  Two  actions  were  here  at 
16 


232  FRAGMENTS    OF    SCIENCE. 

work,  both  tending  to  obliterate  the  lake — the  raising 
of  its  bed  by  the  deposition  of  detritus,  and  the  cutting 
of  its  dam  by  the  river.  In  process  of  time  the  cut 
deepened  into  the  Via  Mala;  the  lake  was  drained,  and 
the  river  now  flows  in  a  definite  channel  through  the 
plain  which  its  waters  once  totally  covered. 

From  Tusis  I  crossed  to  Tiefenkasten  by  the  Schien 
Pass,  and  thence  over  the  Julier  Pass  to  Pontresina. 
There  are  three  or  four  ancient  lake-beds  between 
Tiefenkasten  and  the  summit  of  the  Julier.  They  are 
all  of  the  same  type — a  more  or  less  broad  and  level 
valley-bottom,  with  a  barrier  in  front  through  which 
the  river  has  cut  a  passage,  the  drainage  of  the  lake 
being  the  consequence.  These  lakes  were  sometimes 
dammed  by  barriers  of  rock,  sometimes  by  the  moraines 
of  ancient  glaciers. 

An  example  of  this  latter  kind  occurs  in  the  Rosegg 
valley,  about  twenty  minutes  below  the  end  of  the 
Eosegg  glacier,  and  about  an  hour  from  Pontresina. 
The  valley  here  is  crossed  by  a  pine-covered  moraine 
of  the  noblest  dimensions;  in  the  neighbourhood  of 
London  it  might  be  called  a  mountain.  That  it  is  a 
moraine,  the  inspection  of  it  from  a  point  on  the  Surlei 
slopes  above  it  will  convince  any  person  possessing  an 
educated  eye.  Where,  moreover,  the  interior  of  the 
mound  is  exposed,  it  exhibits  moraine-matter — detritus 
pulverised  by  the  ice,  with  boulders  entangled  in  it. 
It  stretched  quite  across  the  valley,  and  at  one  time 
dammed  the  river  up.  But  now  the  barrier  is  cut 
through,  the  stream  having  about  one-fourth  of  the 
moraine  to  its  right,  and  the  remaining  three-fourths 
to  its  left.  Other  moraines  of  a  more  resisting  charac- 
ter hold  their  ground  as  barriers  to  the  present  day. 
In  the  Val  di  Campo,  for  example,  about  three-quarters 
of  an  hour  from  Pisciadello,  there  is  a  moraine  com- 


ALPINE    SCULPTURE.  233 

posed  of  large  boulders,  which  interrupt  the  course  of  a 
river  and  compel  the  water  to  fall  over  them  in  cas- 
cades. They  have  in  great  part  resisted  its  action  since 
the  retreat  of  the  ancient  glacier  which  formed  the 
moraine.  Behind  the  moraine  is  a  lake-bed,  now  con- 
verted into  a  level  meadow,  which  rests  on  a  deep  layer 
of  mould. 

At  Pontresina  a  very  fine  and  instructive  gorge  is 
to  be  seen.  The  river  from  the  Morteratsch  glacier 
rushes  through  a  deep  and  narrow  chasm  which  is 
spanned  at  one  place  by  a  stone  bridge.  The  rock  is  not 
of  a  character  to  preserve  smooth  polishing;  but  the 
larger  features  of  water-action  are  perfectly  evident 
from  top  to  bottom.  Those  features  are  in  part  visible 
from  the  bridge,  but  still  better  from  a  point  a  little 
distance  from  the  bridge  in  the  direction  of  the  upper 
village  of  Pontresina.  The  hollowing  out  of  the  rock 
by  the  eddies  of  the  water  is  here  quite  manifest.  A 
few  minutes'  walk  upwards  brings  us  to  the  end  of  the 
gorge;  and  behind  it  we  have  the  usual  indications  of 
an  ancient  lake,  and  terraces  of  distinct  water  origin. 
From  this  position  indeed  the  genesis  of  the  gorge  is 
clearly  revealed.  After  the  retreat  of  the  ancient  gla- 
cier, a  transverse  ridge  of  comparatively  resisting  ma- 
terial crossed  the  valley  at  this  place.  Over  the  lowest 
part  of  this  ridge  the  river  flowed,  rushing  steeply 
down  to  join  at  the  bottom  of  the  slope  the  stream 
which  issued  from  the  Rosegg  glacier.  On  this  incline 
the  water  became  a  powerful  eroding  agent,  and  finally 
cut  the  channel  to  its  present  depth. 

Geological  writers  of  reputation  assume  at  this 
place  the  existence  of  a  fissure,  the  '  washing  out '  of 
which  resulted  in  the  formation  of  the  gorge.  Now 
no  examination  of  the  bed  of  the  river  ever  proved  the 
existence  of  this  fissure;  and  it  is  certain  that  water, 


234  FRAGMENTS    OF    SCIENCE. 

particularly  when  charged  with  solid  matter  in  suspen- 
sion, can  cut  a  channel  through  unfissured  rock.  Cases 
of  deep  cutting  can  be  pointed  out  where  the  clean  bed 
of  the  stream  is  exposed,  the  rock  which  forms  the 
floor  of  the  river  not  exhibiting  a  trace  of  fissure.  An 
example  of  this  kind  on  a  small  scale  occurs  near  the 
Bernina  Gasthaus,  about  two  hours  from  Pontresina. 
A  little  way  below  the  junction  of  the  two  streams 
from  the  Bernina  Pass  and  the  Heuthal  the  river  flows 
through  a  channel  cut  by  itself,  and  20  or  30  feet  in 
depth.  At  some  places  the  river-bed  is  covered  with 
rolled  stones;  at  other-places  it  is  bare,  but  shows  no 
trace  of  fissure.  The  abstract  power  of  water,  if  I  may 
use  the  term,  to  cut  through  rock  is  demonstrated  by 
such  instances.  But  if  water  be  competent  to  form  a 
gorge  without  the  aid  of  a  fissure,  why  assume  the  ex- 
istence of  such  fissures  in  cases  like  that  at  Pontresina? 
It  seems  far  more  philosophical  to  accept  the  simple 
and  impressive  history  written  on  the  walls  of  those 
gorges  by  the  agent  which  produced  them. 

Numerous  cases  might  be  pointed  out,  varying  in 
magnitude,  but  all  identical  in  kind,  of  barriers  which 
crossed  valleys  and  formed  lakes  having  been  cut 
through  by  rivers,  narrow  gorges  being  the  conse- 
quence. One  of  the  most  famous  examples  of  this  kind 
is  the  Finsteraarschlucht  in  the  valley  of  Hasli.  Here 
the  ridge  called  the  Kirchet  seems  split  across,  and 
the  river  Aar  rushes  through  the  fissure.  Behind  the 
barrier  we  have  the  meadows  and  pastures  of  Imhof 
resting  on  the  sediment  of  an  ancient  lake.  Were  this 
an  isolated  case,  one  might  with  an  apparent  show  of 
reason  conclude  that  the  Finsteraarschlucht  was  pro- 
duced by  an  earthquake,  as  some  suppose  it  to  have 
been;  but  when  we  find  it  to  be  a  single  sample  of 
actions  which  are  frequent  in  the  Alps — when  prob- 


ALPINE   SCULPTURE.  235 

ably  a  hundred  cases  of  the  same  kind,  though  different 
in  magnitude,  can  be  pointed  out — it  seems  quite  un- 
philosophical  to  assume  that  in  each  particular  case  an 
earthquake  was  at  hand  to  form  a  channel  for  the  river. 
As  in  the  case  of  the  barrier  at  Pontresina,  the  Kirchet, 
after  the  retreat  of  the  Aar  glacier,  dammed  the  waters 
flowing  from  it,  thus  forming  a  lake,  on  the  bed  of 
which  now  stands  the  village  of  Imhof .  Over  this  bar- 
rier the  Aar  tumbled  towards  Meyringen,  cutting,  as 
the  centuries  passed,  its  bed  ever  deeper,  until  finally 
it  became  deep  enough  to  drain  the  lake,  leaving  in  its 
place  the  alluvial  plain,  through  which  the  river  now 
flows  in  a  definite  channel. 

In  1866  I  subjected  the  Finsteraarschlucht  to  a 
close  examination.  The  earthquake  theory  already  ad- 
verted to  was  then  prevalent  regarding  it,  and  I  wished 
to  see  whether  any  evidences  existed  of  aqueous  ero- 
sion. Near  the  summit  of  the  Kirchet  is  a  signboard 
inviting  the  traveller  to  visit  the  Aarenschlucht,  a  nar- 
row lateral  gorge  which  runs  down  to  the  very  bottom 
of  the  principal  one.  The  aspect  of  this  smaller  chasm 
from  bottom  to  top  proves  to  demonstration  that  water 
had  in  former  ages  been  there  at  work.  It  is  scooped, 
rounded,  and  polished,  so  as  to  render  palpable  to 
the  most  careless  eye  that  it  is  a  gorge  of  erosion. 
But  it  was  regarding  the  sides  of  the  great  chasm  that 
instruction  was  needed,  and  from  its  edge  nothing  to 
satisfy  me  could  be  seen.  I  therefore  stripped  and 
waded  into  the  river  until  a  point  was  reached  which 
commanded  an  excellent  view  of  both  sides  of  the 
gorge.  The  water  was  cutting  cold,  but  I  was  repaid. 
Below  me  on  the  left-hand  side  was  a  jutting  cliff 
which  bore  the  thrust  of  the  river  and  caused  the  Aar 
to  swerve  from  its  direct  course.  From  top  to  bottom 
this  cliff  was  polished,  rounded,  and  scooped.  There 


236  FKAGMENTS    OF    SCIENCE. 

was  no  room  for  doubt.  The  river  which  now  runs  so 
deeply  down  had  once  been  above.  It  has  been  the 
delver  of  its  own  channel  through  the  barrier  of  the 
Kirchet. 

But  the  broad  view  taken  by  the  advocates  of  the 
fracture  theory  is,  that  the  valleys  themselves  follow 
the  tracks  of  primeval  fissures  produced  by  the  up- 
heaval of  the  land,  the  cracks  across  the  barriers  re- 
ferred to  being  in  reality  portions  of  the  great  cracks 
which  formed  the  valleys.  Such  an  argument,  how- 
ever, would  virtually  concede  the  theory  of  erosion  as 
applied  to  the  valleys  of  the  Alps.  The  narrow  gorges, 
often  not  more  than  twenty  or  thirty  feet  across,  some- 
times even  narrower,  frequently  occur  at  the  bottom  of 
broad  valleys.  Such  fissures  might  enter  into  the  list 
of  accidents  which  gave  direction  to  the  real  erosive 
agents  which  scooped  the  valley  out;  but  the  formation 
of  the  valley,  as  it  now  exists,  could  no  more  be  as- 
cribed to  such  cracks  than  the  motion  of  a  railway 
train  could  be  ascribed  to  the  finger  of  the  engineer 
which  turns  on  the  steam. 

These  deep  gorges  occur,  I  believe,  for  the  most 
part  in  limestone  strata;  and  the  effects  which  the 
merest  driblet  of  water  can  produce  on  limestone  are 
quite  astonishing.  It  is  not  uncommon  to  meet  chasms 
of  considerable  depth  produced  by  small  streams  the 
beds  of  which  are  dry  for  a  large  portion  of  the  year. 
Eight  and  left  of  the  larger  gorges  such  secondary 
chasms  are  often  found.  The  idea  of  time  must,  I 
think,  be  more  and  more  included  in  our  reasonings  on 
these  phenomena.  Happily,  the  marks  which  the  rivers 
have,  in  most  cases,  left  behind  them,  and  which  refer, 
geologically  considered,  to  actions  of  yesterday,  give  us 
ground  and  courage  to  conceive  what  may  be  effected 
in  geologic  periods.  Thus  the  modern  portion  of  the 


ALPINE    SCULPTURE.  237 

Via  Mala  throws  light  upon  the  whole.  Near  Bergiin, 
in  the  valley  of  the  Albula,  there  is  also  a  little  Via 
Mala,  which  is  not  less  significant  than  the  great  one. 
The  river  flows  here  through  a  profound  limestone 
gorge,  and  to  the  very  edges  of  the  gorge  we  have  the 
evidences  of  erosion.  But  the  most  striking  illustra- 
tion of  water-action  upon  limestone  rock  that  I  have 
ever  seen  is  the  gorge  at  Pfaffere.  Here  the  traveller 
passes  along  the  side  of  the  chasm  midway  between  top 
and  bottom.  Whichever  way  he  looks,  backwards  or 
forwards,  upwards  or  downwards,  towards  the  sky  or 
towards  the  river,  he  meets  everywhere  the  irresistible 
and  impressive  evidence  that  this  wonderful  fissure  has 
been  sawn  through  the  mountain  by  the  waters  of  the 
Tamina. 

I  have  thus  far  confined  myself  to  the  consideration 
of  the  gorges  formed  by  the  cutting  through  of  the 
rock-barriers  which  frequently  cross  the  valleys  of  the 
Alps;  as  far  as  they  have  been  examined  by  me  they 
are  the  work  of  erosion.  But  the  larger  question  still 
remains,  To  what  action  are  we  to  ascribe  the  forma- 
tion of  the  valleys  themselves?  This  question  includes 
that  of  the  formation  of  the  mountain-ridges,  for  were 
the  valleys  wholly  filled,  the  ridges  would  disappear. 
Possibly  no  answer  can  be  given  to  this  question  which 
is  not  beset  with  more  or  less  of  difficulty.  Special 
localities  might  be  found  which  would  seem  to  contra- 
dict every  solution  which  refers  the  conformation  of 
the  Alps  to  the  operation  of  a  single  cause. 

Still  the  Alps  present  features  of  a  character  suffi- 
ciently definite  to  bring  the  question  of  their  origin 
within  the  sphere  of  close  reasoning.  That  they  were 
in  whole  or  in  part  once  beneath  the  sea  will  not  be 
disputed;  for  they  are  in  great  part  composed  of  sedi- 
mentary rocks  which  required  a  sea  to  form  them. 


238  FKAGMENTS    OF    SCIENCE. 

Their  present  elevation  above  the  sea  is  due  to  one  of 
those  local  changes  in  the  shape  of  the  earth  which 
have  been  of  frequent  occurrence  throughout  geologic 
time,  in  some  cases  depressing  the  land,  and  in  others 
causing  the  sea-bottom  to  protrude  beyond  its  surface. 
Considering  the  inelastic  character  of  its  materials,  the 
protuberance  of  the  Alps  could  hardly  have  been 
pushed  out  without  dislocation  and  fracture;  and  this 
conclusion  gains  in  probability  when  we  consider  the 
foldings,  contortions,  and  even  reversals  in  position  of 
the  strata  in  many  parts  of  the  Alps.  Such  changes 
in  the  position  of  beds  which  were  once  horizontal 
could  not  have  been  effected  without  dislocation.  Fis- 
sures would  be  produced  by  these  changes;  and  such 
fissures,  the  advocates  of  the  fracture  theory  contend, 
mark  the  positions  of  the  valleys  of  the  Alps. 

Imagination  is  necessary  to  the  man  of  science,  and 
we  could  not  reason  on  our  present  subject  without  the 
power  of  presenting  mentally  a  picture  of  the  earth's 
crust  cracked  and  fissured  by  the  forces  which  pro- 
duced its  upheaval.  Imagination,  however,  must  be 
strictly  checked  by  reason  and  by  observation.  That 
fractures  occurred  cannot,  I  think,  be  doubted,  but 
that  the  valleys  of  the  Alps  are  thus  formed  is  a  conclu- 
sion not  at  all  involved  in  the  admission  of  dislocations. 
I  never  met  with  a  precise  statement  of  the  manner  in 
which  the  advocates  of  the  fissure  theory  suppose  the 
forces  to  have  acted — whether  they  assume  a  general 
elevation  of  the  region,  or  a  local  elevation  of  distinct 
ridges;  or  whether  they  assume  local  subsidences  after 
a  general  elevation,  or  whether  they  would  superpose 
upon  the  general  upheaval  minor  and  local  upheavals. 

In  the  absence  of  any  distinct  statement,  I  will  as- 
sume the  elevation  to  be  general — that  a  swelling  out 
of  the  earth's  crust  occurred  here,  sufficient  to  place 


ALPINE    SCULPTURE.  239 

the  most  prominent  portions  of  the  protuberance  three 
miles  above  the  sea-level.  To  fix  the  ideas,  let  us  con- 
sider a  circular  portion  of  the  crust,  say  one  hundred 
miles  in  diameter,  and  let  us  suppose,  in  the  first  in- 
stance, the  circumference  of  this  circle  to  remain  fixed, 
and  that  the  elevation  was  confined  to  the  space  within 
it.  The  upheaval  would  throw  the  crust  into  a  state 
of  strain;  and,  if  it  were  inflexible,  the  strain  must  be 
relieved  by  fracture.  Crevasses  would  thus  intersect 
the  crust.  Let  us  now  enquire  what  proportion  the 
area  of  these  open  fissures  is  likely  to  bear  to  the  area 
of  the  unfissured  crust.  An  approximate  answer  is  all 
that  is  here  required;  for  the  problem  is  of  such  a 
character  as  to  render  minute  precision  unnecessary. 

No  one,  I  think,  would  affirm  that  the  area  of  the 
fissures  would  be  one-hundredth  the  area  of  the  land. 
For  let  us  consider  the  strain  upon  a  single  line  drawn 
over  the  summit  of  the  protuberance  from  a  point  on 
its  rim  to  a  point  opposite.  Regarding  the  protuber- 
ance as  a  spherical  swelling,  the  length  of  the  arc  cor- 
responding to  a  chord  of  100  miles  and  a  versed  sine 
of  3  miles  is  100.24  miles;  consequently  the  surface  to 
reach  its  new  position  must  stretch  0.24  of  a  mile,  or 
be  broken.  A  fissure  or  a  number  of  cracks  with  this 
total  width  would  relieve  the  strain;  that  is  to  say,  the 
sum  of  the  widths  of  all  the  cracks  over  the  length  of 
100  miles  would  be  420  yards.  If,  instead  of  com- 
paring the  width  of  the  fissures  with  the  length  of  the 
lines  of  tension,  we  compared  their  areas  with  the  area 
of  the  unfissured  land,  we  should  of  course  find  the 
proportion  much  less.  These  considerations  will  help 
the  imagination  to  realise  what  a  small  ratio  the  area 
of  the  open  fissures  must  bear  to  the  unfissured  crust. 
They  enable  us  to  say,  for  example,  that  to  assume  the 
area  of  the  fissures  to  be  one-tenth  of  the  area  of  the 


240  FKAGMENTS    OF    SCIENCE. 

land  would  be  quite  absurd,  while  that  the  area  of  the 
fissures  could  be  one-half  or  more  than  one-half  that  of 
the  land  would  be  in  a  proportionate  degree  unthink- 
able. If  we  suppose  the  elevation  to  be  due  to  the 
shrinking  or  subsidence  of  the  land  all  round  our  as- 
sumed circle,  we  arrive  equally  at  the  conclusion  that 
the  area  of  the  open  fissures  would  be  altogether  in- 
significant as  compared  with  that  of  the  unfissured 
crust. 

To  those  who  have  seen  them  from  a  commanding 
elevation,  it  is  needless  to  say  that  the  Alps  themselves 
bear  no  sort  of  resemblance  to  the  picture  which  this 
theory  presents  to  us.  Instead  of  deep  cracks  with 
approximately  vertical  walls,  we  have  ridges  running 
into  peaks,  and  gradually  sloping  to  form  valleys.  In- 
stead of  a  fissured  crust,  we  have  a  state  of  things 
closely  resembling  the  surface  of  the  ocean  when  agi- 
tated by  a  storm.  The  valleys,  instead  of  being  much 
narrower  than  the  ridges,  occupy  the  greater  space.  A 
plaster  cast  of  the  Alps  turned  upside  down,  so  as  to 
invert  the  elevations  and  depressions,  would  exhibit 
blunter  and  broader  mountains,  with  narrower  valleys 
between  them,  than  the  present  ones.  The  valleys  that 
exist  cannot,  I  think,  with  any  correctness  of  language 
be  called  fissures.  It  may  be  urged  that  they  origi- 
nated in  fissures:  but  even  this  is  unproved,  and,  were 
it  proved,  the  fissures  would  still  play  the  subordinate 
part  of  giving  direction  to  the  agents  which  are  to  be 
regarded  as  the  real  sculptors  of  the  Alps. 

The  fracture  theory,  then,  if  it  regards  the  eleva- 
tion of  the  Alps  as  due  to  the  operation  of  a  force  act- 
ing throughout  the  entire  region,  is,  in  my  opinion, 
utterly  incompetent  to  account  for  the  conformation 
of  the  country.  If,  on  the  other  hand,  we  are  com- 
pelled to  resort  to  local  disturbances,  the  manipulation 


ALPINE    SCULPTURE.  241 

of  the  earth's  crust  necessary  to  obtain  the  valleys  and 
the  mountains  will,  I  imagine,  bring  the  difficulties  of 
the  theory  into  very  strong  relief.  Indeed  an  examina- 
tion of  the  region  from  many  of  the  more  accessible 
eminences — from  the  Galenstock,  the  Grauhaupt,  the 
Pitz  Languard,  the  Monte  Confinale — or,  better  still, 
from  Mont  Blanc,  Monte  Eosa,  the  Jungfrau,  the 
Finsteraarhorn,  the  Weisshorn,  or  the  Matterhorn, 
where  local  peculiarities  are  toned  down,  and  the  op- 
erations of  the  powers  which  really  made  this  region 
what  it  is  are  alone  brought  into  prominence — must, 
I  imagine,  convince  every  physical  geologist  of  the  in- 
ability of  any  fracture  theory  to  account  for  the  pres- 
ent conformation  of  the  Alps. 

A  correct  model  of  the  mountains,  with  an  un- 
exaggerated  vertical  scale,  produces  the  same  effect 
upon  the  mind  as  the  prospect  from  one  of  the  highest 
peaks.  We  are  apt  to  be  influenced  by  local  phenom- 
ena which,  though  insignificant  in  view  of  the  general 
question  of  Alpine  conformation,  are,  with  reference  to 
our  customary  standards,  vast  and  impressive.  In  a 
true  model  those  local  peculiarities  disappear;  for  on 
the  scale  of  a  model  they  are  too  small  to  be  visible; 
while  the  essential  facts  and  forms  are  presented  to 
the  undistracted  attention. 

A  minute  analysis  of  the  phenomena  strengthens 
the  conviction  which  the  general  aspect  of  the  Alps 
fixes  in  the  mind.  We  find,  for  example,  numerous 
valleys  which  the  most  ardent  plutonist  would  not 
think  of  ascribing  to  any  other  agency  than  erosion. 
That  such  is  their  genesis  and  history  is  as  certain  as 
that  erosion  produced  the  Chines  in  the  Isle  of  Wight. 
From  these  indubitable  cases  of  erosion — commencing, 
if  necessary,  with  the  small  ravines  which  run  down 
the  flanks  of  the  ridges,  with  their  little  working 


242  FKAGMENTS    OF    SCIENCE. 

navigators  at  their  bottoms — we  can  proceed,  by  almost 
insensible  gradations,  to  the  largest  valleys  of  the  Alps; 
and  it  would  perplex  the  phitonist  to  fix  upon  the 
point  at  which  fracture  begins  to  play  a  material  part. 

In  ascending  one  of  the  larger  valleys,  we  enter  it 
where  it  is  wide  and  where  the  eminences  are  gentle  on 
either  side.  The  flanking  mountains  become  higher 
and  more  abrupt  as  we  ascend,  and  at  length  we  reach 
a  place  where  the  depth  of  the  valley  is  a  maximum. 
Continuing  our  walk  upwards,  we  find  ourselves 
flanked  by  gentler  slopes,  and  finally  emerge  from  the 
valley  and  reach  the  summit  of  an  open  col,  or  de- 
pression in  the  chain  of  mountains.  This  is  the  com- 
mon character  of  the  large  valleys.  Crossing  the  col, 
we  descend  along  the  opposite  slope  of  the  chain,  and 
through  the  same  series  of  appearances  in  the  reverse 
order.  If  the  valleys  on  both  sides  of  the  col  were  pro- 
duced by  fissures,  what  prevents  the  fissure  from  pro- 
longing itself  across  the  col?  The  case  here  cited  is 
representative;  and  I  am  not  acquainted  with  a  single 
instance  in  the  Alps  where  the  chain  has  been  cracked 
in  the  manner  indicated.  The  cols  are  simply  de- 
pressions; in  many  of  which  the  unfissured  rock  can 
be  traced  from  side  to  side. 

The  typical  instance  just  sketched  follows  as  a 
natural  consequence  from  the  theory  of  erosion.  Be- 
fore either  ice  or  water  can  exert  great  power  as  an 
erosive  agent,  it  must  collect  in  sufficient  mass.  On 
the  higher  slopes  and  plateaus — in  the  region  of  cols — 
the  power  is  not  fully  developed;  but  lower  down  tribu- 
taries unite,  erosion  is  carried  on  with  increased  vigour, 
and  the  excavation  gradually  reaches  a  maximum. 
Lower  still  the  elevations  diminish  and  the  slopes  be- 
come more  gentle;  the  cutting  power  gradually  re- 
laxes, until  finally  the  eroding  agent  quits  the  moun- 


ALPINE    SCULPTURE.  243 

tains  altogether,  and  the  grand  effects  which  it  pro- 
duced in  the  earlier  portions  of  its  course  entirely  dis- 
appear. 

I  have  hitherto  confined  myself  to  the  consideration 
of  the  broad  question  of  the  erosion  theory  as  compared 
with  the  fracture  theory;  and  all  that  I  have  been  able 
to  observe  and  think  with  reference  to  the  subject 
leads  me  to  adopt  the  former.  Under  the  term  erosion 
I  include  the  action  of  water,  of  ice,  and  of  the  at- 
mosphere, including  frost  and  rain.  Water  and  ice, 
however,  are  the  principal  agents,  and  which  of  these 
two  has  produced  the  greatest  effect  it  is  perhaps  im- 
possible to  say.  Two  years  ago  I  wrote  a  brief  note 
(  On  the  Conformation  of  the  Alps,'  *  in  which  I  as- 
cribed the  paramount  influence  to  glaciers.  The  facts 
on  which  that  opinion  was  founded  are,  I  think,  un- 
assailable; but  whether  the  conclusion  then  announced 
fairly  follows  from  the  facts  is,  I  confess,  an  open 
question. 

The  arguments  which  have  been  thus  far  urged 
against  the  conclusion  are  not  convincing.  Indeed, 
the  idea  of  glacier  erosion  appear  so  daring  to  some 
minds  that  its  boldness  alone  is  deemed  its  sufficient 
refutation.  It  is,  however,  to  be  remembered  that  a 
precisely  similar  position  was  taken  up  by  many  ex- 
cellent workers  when  the  question  of  ancient  glacier 
extension  was  first  mooted.  The  idea  was  considered 
too  hardy  to  be  entertained;  and  the  evidences  of 
glacial  action  were  sought  to  be  explained  by  reference 
to  almost  any  process  rather  than  the  true  one.  Let 
those  who  so  wisely  took  the  side  of  *  boldness '  in  that 
discussion  beware  lest  they  place  themselves,  with 
reference  to  the  question  of  glacier  erosion,  in  the 
position  formerly  occupied  by  their  opponents. 
*  Phil.  Mag.  vol.  xxir.  p.  169. 


244  FRAGMENTS    OF    SCIENCE. 

Looking  at  the  little  glaciers  of  the  present  day — 
mere  pigmies  as  compared  to  the  giants  of  the  glacial 
epoch — we  find  that  from  every  one  of  them  issues  a 
river  more  or  less  voluminous,  charged  with  the  matter 
which  the  ice  has  rubbed  from  the  rocks.  Where  the 
rocks  are  soft,  the  amount  of  this  finely  pulverised 
matter  suspended  in  the  water  is  very  great.  The 
water,  for  example,  of  the  river  which  flows  from 
Santa  Catarina  to  Bormio  is  thick  with  it.  The  Ehine 
is  charged  with  this  matter,  and  by  it  has  so  silted  up 
the  Lake  of  Constance  as  to  abolish  it  for  a  large  frac- 
tion of  its  length.  The  Rhone  is  charged  with  it,  and 
tens  of  thousands  of  acres  of  cultivable  land  are  formed 
by  the  silt  above  the  Lake  of  Geneva. 

In  the  case  of  every  glacier  we  have  two  agents  at 
work — the  ice  exerting  a  crushing  force  on  every  point 
of  its  bed  which  bears  its  weight,  and  either  rasping 
this  point  into  powder  or  tearing  it  bodily  from  the 
rock  to  which  it  belongs;  while  the  water  which  every- 
where circulates  upon  the  bed  of  the  glacier  continually 
washes  the  detritus  away  and  leaves  the  rock  clean  for 
further  abrasion.  Confining  the  action  of  glaciers  to 
the  simple  rubbing  away  of  the  rocks,  and  allowing 
them  sufficient  time  to  act,  it  is  not  a  matter  of  opinion, 
but  a  physical  certainty,  that  they  will  scoop  out  val- 
leys. But  the  glacier  does  more  than  abrade.  Rocks 
are  not  homogeneous;  they  are  intersected  by  joints 
and  places  of  weakness,  which  divide  them  into  vir- 
tually detached  masses.  A  glacier  is  undoubtedly  com- 
petent to  root  such  masses  bodily  away.  Indeed  the 
mere  a  priori  consideration  of  the  subject  proves  the 
competence  of  a  glacier  to  deepen  its  bed.  Taking  the 
case  of  a  glacier  1,000  feet  deep  (and  some  of  the  older 
ones  were  probably  three  times  this  depth),  and  allow- 
ing 40  feet  of  ice  to  an  atmosphere,  we  find  that  on 


ALPINE    SCULPTURE.  245 

every  square  inch  of  its  bed  such  a  glacier  presses  with 
a  weight  of  375  Ibs.,  and  on  every  square  yard  of  its 
bed  with  a  weight  of  486,000  Ibs.  With  a  vertical  pres- 
sure of  this  amount  the  glacier  is  urged  down  its  valley 
by  the  pressure  from  behind.  We  can  hardly,  I  think, 
deny  to  such  a  tool  a  power  of  excavation. 

The  retardation  of  a  glacier  by  its  bed  has  been 
referred  to  as  proving  its  impotence  as  an  erosive 
agent;  but  this  very  retardation  is  in  some  measure  an 
expression  of  the  magnitude  of  the  erosive  energy. 
Either  the  bed  must  give  way,  or  the  ice  must  slide 
over  itself.  We  get  indeed  some  idea  of  the  crushing 
pressure  which  the  moving  glacier  exercises  against  its 
bed  from  the  fact  that  the  resistance,  and  the  effort  to 
overcome  it,  are  such  as  to  make  the  upper  layers  of  a 
glacier  move  bodily  over  the  lower  ones — a  portion 
only  of  the  total  motion  being  due  to  the  progress  of 
the  entire  mass  of  the  glacier  down  its  valley. 

The  sudden  bend  in  the  valley  of  the  Ehone  at 
Martigny  has  also  been  regarded  as  conclusive  evidence 
against  the  theory  of  erosion.  '  Why,'  it  has  been 
asked,  '  did  not  the  glacier  of  the  Rhone  go  straight 
forward  instead  of  making  this  awkward  bend? '  But 
if  the  valley  be  a  crack,  why  did  the  crack  make  this 
bend?  The  crack,  I  submit,  had  at  least  as  much 
reason  to  prolong  itself  in  a  straight  line  as  the  glacier 
had.  A  statement  of  Sir  John  Herschel  with  reference 
to  another  matter  is  perfectly  applicable  here:  'A 
crack  once  produced  has  a  tendency  to  run — for  this 
plain  reason,  that  at  its  momentary  limit,  at  the  point 
at  which  it  has  just  arrived,  the  divellent  force  on  the 
molecules  there  situated  is  counteracted  only  by  half 
of  the  cohesive  force  which  acted  when  there  was  no 
crack,  viz.  the  cohesion  of  the  uncracked  portion  alone' 
('  Proc.  Roy.  Soc.'  vol.  xii.  p.  678).  To  account,  then, 


246  FEAGMENTS    OF    SCIENCE. 

for  the  bend,  the  adherent  of  the  fracture  theory  must 
assume  the  existence  of  some  accident  which  turned 
the  crack  at  right  angles  to  itself;  and  he  surely  will 
permit  the  adherent  of  the  erosion  theory  to  make  a 
similar  assumption. 

The  influence  of  small  accidents  on  the  direction  of 
rivers  is  beautifully  illustrated  in  glacier  streams, 
which  are  made  to  cut  either  straight  or  sinuous  chan- 
nels by  causes  apparently  of  the  most  trivial  character. 
In  his  interesting  paper  '  On  the  Lakes  of  Switzerland,' 
M.  Studer  also  refers  to  the  bend  of  the  Ehine  at 
Sargans  in  proof  that  the  river  must  there  follow  a 
pre-existing  fissure.  I  made  a  special  expedition  to  the 
place  in  1864;  and  though  it  was  plain  that  M.  Studer 
had  good  grounds  for  the  selection  of  this  spot,  I  was 
unable  to  arrive  at  his  conclusion  as  to  the  necessity 
of  a  fissure. 

Again,  in  the  interesting  volume  recently  published 
by  the  Swiss  Alpine  Club,  M.  Desor  informs  us  that 
the  Swiss  naturalists  who  met  last  year  at  Samaden 
visited  the  end  of  the  Morteratsch  glacier,  and  there 
convinced  themselves  that  a  glacier  had  no  tendency 
whatever  to  imbed  itself  in  the  soil.  I  scarcely  think 
that  the  question  of  glacier  erosion,  as  applied  either  to 
lakes  or  valleys,  is  to  be  disposed  of  so  easily.  Let  me 
record  here  my  experience  of  the  Morteratsch  glacier. 
I  took  with  me  in  1864  a  theodolite  to  Pontresina, 
and  while  there  had  to  congratulate  myself  on  the  aid 
of  my  friend  Mr.  Hirst,  who  in  1857  did  such  good 
service  upon  the  Mer  de  Glace  and  its  tributaries.  We 
set  out  three  lines  across  the  Morteratsch  glacier,  one 
of  which  crossed  the  ice-stream  near  the  well-known 
hut  of  the  painter  Georgei,  while  the  two  others  were 
staked  out,  the  one  above  the  hut  and  the  other  below 
it.  Calling  the  highest  line  A,  the  line  which  crossed 


ALPINE    SCULPTURE. 


247 


the  glacier  at  the  hut  B,  and  the  lowest  line  C,  the  fol- 
lowing are  the  mean  hourly  motions  of  the  three  lines, 
deduced  from  observations  which  extended  over  several 
days.  On  each  line  eleven  stakes  were  fixed,  which  are 
designated  by  the  figures  1,  2,  3,  &c.  in  the  Tables. 
Morteratsch  Glacier,  Line  A. 


No.  of  Stake. 
1     . 


Hourly  Motion. 

.  0.35  inch. 

.  0.49 

.  0.53 

.  0.54 

.  0.56 

.  0.54 

.  0.52 

.  0.49 

.  0.40 

.  0.29 

.  0.20 


As  in  all  other  measurements  of  this  kind,  the  re- 
tarding influence  of  the  sides  of  the  glacier  is  manifest: 
the  centre  moves  with  the  greatest  velocity. 


No.  of  Stake. 


Morteratsch  Glacier,  Line  B. 

Hourly  Motion. 

.        .        .        .    0.05  inch. 

.        .        I        .    0.14 

.        .        .    0.24 

.        .        .       ,    *  *&     .    0.32 

.        .    .    *..     .    0.41 

.       .0.44 

;       .       .0.44 

.    0.45 

r.: 0.43 

^ ...       .       .       .        .        .0.44 

.    0.44 


The  first  stake  of  this  line  was  quite  close  to  the 

edge  of  the  glacier,  and  the  ice  was  thin  at  the  place, 

hence  its  slow  motion.    Crevasses  prevented  us  from 

carrying  the  line  sufficiently  far  across  to  render  the 

17 


248  FRAGMENTS    OF    SCIENCE. 

retardation  of  the  further  side  of  the  glacier  fully 
evident. 

Morteratsch  Glacier,  Line  C. 
No.  of  Stake.  Hourly  Motion. 

1 0.05  inch. 

2 0.09     " 

3 0.18    " 

4    ........    0.20    " 

5 0.25    " 

C .        .    0.27    " 

7 ..  0.27    " 

0 .        .     0.30     " 

9     .       •-..      '.       -.        .        .        .        .     0.21     " 

10 0.20     " 

11 0.16    " 

Comparing  the  three  lines  together,  it  will  be  ob- 
served that  the  velocity  diminishes  as  we  descend  the 
glacier.  In  100  hours  the  maximum  motion  of  the 
three  lines  respectively  is  as  follows: 

Maximum  Motion  in  100  hours. 

Line  A .56  inches. 

"     B -  .        .     45       " 

"    C     .        .        .        ...        .    30      " 

This  deportment  explains  an  appearance  which 
must  strike  every  observer  who  looks  upon  the  Mor- 
teratsch  from  the  Piz  Languard,  or  from  the  new 
Bernina  Eoad.  A  medial  moraine  runs  along  the 
glacier,  commencing  as  a  narrow  streak,  but  towards 
the  end  the  moraine  extending  in  width,  until  finally 
it  quite  covers  the  terminal  portion  of  the  glacier. 
The  cause  of  this  is  revealed  by  the  foregoing  measure- 
ments, which  prove  that  a  stone  on  the  moraine  where 
it  is  crossed  by  the  line  A  approaches  a  second  stone 
on  the  moraine  where  it  is  crossed  by  the  line  C  with  a 
velocity  of  twenty-six  inches  per  one  hundred  hours. 
The  moraine  is  in  a  state  of  longitudinal  compression. 
Its  materials  are  more  and  more  squeezed  together, 


ALPINE    SCULPTURE.  249 

and  they  must  consequently  move  laterally  and  render 
the  moraine  at  the  terminal  portion  of  the  glacier 
wider  than  above. 

The  motion  of  the  Morteratsch  glacier,  then,  di- 
minishes as  we  descend.  The  maximum  motion  of 
the  third  line  is  thirty  inches  in  one  hundred  hours,  or 
seven  inches  a  day — a  very  slow  motion;  and  had  we 
run  a  line  nearer  to  the  end  of  the  glacier,  the  motion 
would  have  been  slower  still.  At  the  end  itself  it  is 
nearly  insensible.*  Now  I  submit  that  this  is  not  the 
place  to  seek  for  the  scooping  power  of  a  glacier.  The 
opinion  appears  to  be  prevalent  that  it  is  the  snout  of 
a  glacier  that  must  act  the  part  of  ploughshare;  and  it 
is  certainly  an  erroneous  opinion.  The  scooping  power 
will  exert  itself  most  where  the  weight  and  the  motion 
are  greatest.  A  glacier's  snout  often  rests  upon  matter 
which  has  been  scooped  from  the  glacier's  bed  higher 
up.  I  therefore  do  not  think  that  the  inspection  of 
what  the  end  of  a  glacier  does  or  does  not  accomplish 
can  decide  this  question. 

The  snout  of  a  glacier  is  potent  to  remove  anything 
against  which  it  can  fairly  abut;  and  this  power,  not- 
withstanding the  slowness  of  the  motion,  manifests 
itself  at  the  end  of  the  Morteratsch  glacier.  A  hillock, 
bearing  pine-trees,  was  in  front  of  the  glacier  when 
Mr.  Hirst  and  myself  inspected  its  end;  and  this  hillock 
is  being  bodily  removed  by  the  thrust  of  the  ice.  Sev- 
eral of  the  trees  are  overturned;  and  in  a  few  years, 
if  the  glacier  continues  its  reputed  advance,  the  mound 
will  certainly  be  ploughed  away. 

The  question  of  Alpine  conformation   stands,  I 

*  The  snout  of  the  Aletsch  glacier  has  a  diurnal  motion  of 
less  than  two  inches,  while  a  mile  or  so  above  the  snout  the  ve- 
locity is  eighteen  inches.  The  spreading  out  of  the  moraine  is 
here  very  striking. 


250  FRAGMENTS    OF    SCIENCE. 

think,  thus:  We  have,  in  the  first  place,  great  valleys, 
such  as  those  of  the  Ehine  and  the  Shone,  which  we 
might  conveniently  call  valleys  of  the  first  order.  The 
mountains  which  flank  these  main  valleys  are  also  cut 
by  lateral  valleys  running  into  the  main  ones,  and 
which  may  be  called  valleys  of  the  second  order.  When 
these  latter  are  examined,  smaller  valleys  are  found 
running  into  them,  which  may  be  called  valleys  of  the 
third  order.  Smaller  ravines  and  depressions,  again, 
join  the  latter,  which  may  be  called  valleys  of  the 
fourth  order,  and  so  on  until  we  reach  streaks  and  cut- 
tings so  minute  as  not  to  merit  the  name  of  valleys  at 
all.  At  the  bottom  of  every  valley  we  have  a  stream, 
diminishing  in  magnitude  as  the  order  of  the  valley 
ascends,  carving  the  earth  and  carrying  its  materials 
to  lower  levels.  We  find  that  the  larger  valleys  have 
been  filled  for  untold  ages  by  glaciers  of  enormous  di- 
mensions, always  moving,  grinding  down  and  tearing 
away  the  rocks  over  which  they  passed.  We  have, 
moreover,  on  the  plains  at  the  feet  of  the  mountains, 
and  in  enormous  quantities,  the  very  matter  derived 
from  the  sculpture  of  the  mountains  themselves. 

The  plains  of  Italy  and  Switzerland  are  cumbered 
by  the  debris  of  the  Alps.  The  lower,  wider,  and 
more  level  valleys  are  also  filled  to  unknown  depths 
with  the  materials  derived  from  the  higher  ones.  In 
the  vast  quantities  of  moraine-matter  which  cumber 
many  even  of  the  higher  valleys  we  have  also  sugges- 
tions as  to  the  magnitude  of  the  erosion  which  has 
taken  place.  This  moraine-matter,  moreover,  can  only 
in  small  part  have  been  derived  from  the  falling  of 
rocks  upon  the  ancient  glacier;  it  is  in  great  part 
derived  from  the  grinding  and  the  ploughing-out  of 
the  glacier  itself.  This  accounts  for  the  magnitude  of 
many  of  the  ancient  moraines,  which  date  from  a 


ALPINE    SCULPTURE.  251 

period  when  almost  all  the  mountains  were  covered 
with  ice  and  snow,  and  when,  consequently,  the  quan- 
tity of  moraine-matter  derived  from  the  naked  crests 
cannot  have  been  considerable. 

The  erosion  theory  ascribes  the  formation  of  Alpine 
valleys  to  the  agencies  here  briefly  referred  to.  It 
invokes  nothing  but  true  causes.  Its  artificers  are 
still  there,  though,  it  may  be,  in  diminished  strength; 
and  if  they  are  granted  sufficient  time,  it  is  demon- 
strable that  they  are  competent  to  produce  the  effects 
ascribed  to  them.  And  what  does  the  fracture  theory 
offer  in  comparison?  From  no  possible  application  of 
this  theory,  pure  and  simple,  can  we  obtain  the  slopes 
and  forms  of  the  mountains.  Erosion  must  in  the 
long  run  be  invoked,  and  its  power  therefore  conceded. 
The  fracture  theory  infers  from  the  disturbances  of  the 
Alps  the  existence  of  fissures;  and  this  is  a  probable 
inference.  But  that  they  were  of  a  magnitude  suffi- 
cient to  produce  the  conformation  of  the  Alps,  and 
that  they  followed,  as  the  Alpine  valleys  do,  the  lines 
of  natural  drainage  of  the  country,  are  assumptions 
which  do  not  appear  to  me  to  be  justified  either  by 
reason  or  by  observation. 

There  is  a  grandeur  in  the  secular  integration  of 
small  effects  implied  by  the  theory  of  erosion  almost 
superior  to  that  involved  in  the  idea  of  a  cataclysm. 
Think  of  the  ages  which  must  have  been  consumed  in 
the  execution  of  this  colossal  sculpture.  The  question 
may,  of  course,  be  pushed  further.  Think  of  the  ages 
which  the  molten  earth  required  for  its  consolidation. 
But  these  vaster  epochs  lack  sublimity  through  our 
inability  to  grasp  them.  They  bewilder  us,  but  they 
fail  to  make  a  solemn  impression.  The  genesis  of  the 
mountains  comes  more  within  the  scope  of  the  intel- 
lect, and  the  majesty  of  the  operation  is  enhanced  by 


252  FRAGMENTS    OF    SCIENCE. 

our  partial  ability  to  conceive  it.  In  the  falling  of  a 
rock  from  a  mountain-head,  in  the  shoot  of  an  ava- 
lanche, in  the  plunge  of  a  cataract,  we  often  see  more 
impressive  illustrations  of  the  power  of  gravity  than  in 
the  motions  of  the  stars.  When  the  intellect  has  to 
intervene,  and  calculation  is  necessary  to  the  building 
up  of  the  conception,  the  expansion  of  the  feelings 
ceases  to  be  proportional  to  the  magnitude  of  the  phe- 
nomena. 


I  will  here  record  a  few  other  measurements  exe- 
cuted on  the  Rosegg  glacier:  the  line  was  staked  out 
across  the  trunk  formed  by  the  junction  of  the  Rosegg 
proper  with  the  Tschierva  glacier,  a  short  distance 
below  the  rocky  promontory  called  Agaliogs. 

Rosegg  Glacier. 
No.  of  Stake.  Hourly  Motion. 

1 0.01  inch. 

2 0.05  " 

3  0.07  " 

4 0.10  " 

5 0.11  " 

6 0.13  " 

7 0.14  " 

8  .        .        .        .        ...        .  0.18  " 

9  .        .        .        .        ...        .  0.24  " 

10  .'-.--.'.        .      ...        .        .  0.23  " 

11 0.24  " 

This  is  an  extremely  slowly  moving  glacier;  the 
maximum  motion  hardly  amounts  to  seven  inches  a 
day.  Crevasses  prevented  us  from  continuing  the  line 
quite  across  the  glacier. 


X. 

RECENT  EXPERIMENTS  ON  FOG-SIGNALS* 


HE  care  of  its  sailors  is  one  of  the  first  duties  of  a 
-L  maritime  people,  and  one  of  the  sailor's  greatest 
dangers  is  his  proximity  to  the  coast  at  night.  Hence 
the  idea  of  warning  him  of  such  proximity  by  beacon- 
fires  placed  sometimes  on  natural  eminences  and  some- 
times on  towers  built  expressly  for  the  purpose.  Close 
to  Dover  Castle,  for  example,  stands  an  ancient  Pharos 
of  this  description. 

As  our  marine  increased  greater  skill  was  invoked, 
and  lamps  reinforced  by  parabolic  reflectors  poured 
their  light  upon  the  sea.  Several  of  these  lamps  were 
sometimes  grouped  together  so  as  to  intensify  the  light, 
which  at  a  little  distance  appeared  as  if  it  emanated 
from  a  single  source.  This  '  catoptric  '  form  of  appa- 
ratus is  still  to  some  extent  employed  in  our  light- 
house-service, but  for  a  long  time  past  it  has  been 
more  and  more  displaced  by  the  great  lenses  devised  by 
the  illustrious  Frenchman,  Fresnel. 

In  a  first-class  '  dioptric  '  apparatus  the  light  ema- 
nates from  a  lamp  with  several  concentric  wicks,  the 
flame  of  which,  being  kindled  by  a  very  active  draught, 
attains  to  great  intensity.  In  fixed  lights  the  lenses 
refract  the  rays  issuing  from  the  lamp  so  as  to  cause 
them  to  form  a  luminous  sheet  which  grazes  the  sea- 

*  A  discourse  delivered  in  the  Royal  Institution,  March  2?, 
1878. 

253 


254  FRAGMENTS    OF    SCIENCE. 

horizon.  In  revolving  lights  the  lenses  gather  up  the 
rays  into  distinct  beams,  resembling  the  spokes  of  a 
wheel,  which  sweep  over  the  sea  and  strike  the  eye  of 
the  mariner  in  succession. 

It  is  not  for  clear  weather  that  the  greatest 
strengthening  of  the  light  is  intended,  for  here  it  is 
not  needed.  Nor  is  it  for  densely  foggy  weather,  for 
here  it  is  ineffectual.  But  it  is  for  the  intermediate 
stages  of  hazy,  snowy,  or  rainy  weather,  in  which  a 
powerful  light  can  assert  itself,  while  a  feeble  one  is 
extinguished.  The  usual  first-order  lamp  is  one  of 
four  wicks,  but  Mr.  Douglass,  the  able  and  indefatiga- 
ble engineer  of  the  Trinity  House,  has  recently  raised 
the  number  of  the  wicks  to  six,  which  produce  a  very 
noble  flame.  To  Mr.  Wigham,  of  Dublin,  we  are  in- 
debted for  the  successful  application  of  gas  to  light- 
house illumination.  In  some  lighthouses  his  power 
varies  from  28  jets  to  108  jets,  while  in  the  lighthouse 
of  Galley  Head  three  burners  of  the  largest  size  can  be 
employed,  the  maximum  number  of  jets  being  324. 
These  larger  powers  are  invoked  only  in  case  of  fog, 
the  28-jet  burner  being  amply  sufficient  for  clear  wea- 
ther. The  passage  from  the  small  burner  to  the  large, 
and  from  the  large  burner  to  the  small,  is  made  with 
ease,  rapidity,  and  certainty.  This  employment  of  gas 
is  indigenous  to  Ireland,  and  the  Board  of  Trade  has 
exercised  a  wise  liberality  in  allowing  every  facility 
to  Mr.  Wigham  for  the  development  of  his  invention. 

The  last  great  agent  employed  in  lighthouse  illu- 
mination is  electricity.  It  was  in  this  Institution,  be- 
ginning in  1831,  that  Faraday  proved  the  existence 
and  illustrated  the  laws  of  those  induced  currents 
which  in  our  day  have  received  such  astounding  de- 
velopment. In  relation  to  this  subject  Faraday's  words 
have  a  prophetic  ring.  '  I  have  rather,'  he  writes  in 


RECENT    EXPERIMENTS    ON    FOG-SIGNALS.     255 

1831, '  been  desirous  of  discovering  new  facts  and  new 
relations  dependent  on  magneto-electric  induction 
than  of  exalting  the  force  of  those  already  obtained, 
being  assured  that  the  latter  would  find  their  full  de- 
velopment hereafter.'  The  labours  of  Holmes,  of  the 
Paris  Alliance  Company,  of  Wilde,  and  of  Gramme, 
constitute  a  brilliant  fulfilment  of  this  prediction. 

But,  as  regards  the  augmentation  of  power,  the 
greatest  step  hitherto  made  was  independently  taken 
a  few  years  ago  by  Dr.  Werner  Siemens  and  Sir  Charles 
Wheatstone.  Through  the  application  of  their  dis- 
covery a  machine  endowed  with  an  infinitesimal  charge 
of  magnetism  may,  by  a  process  of  accumulation  at 
compound  interest,  be  caused  so  to  enrich  itself  mag- 
netically as  to  cast  by  its  performance  all  the  older 
machines  into  the  shade.  '  The  light  now  before  you  is 
that  of  a  small  machine  placed  downstairs,  and  worked 
there  by  a  minute  steam-engine.  It  is  a  light  of  about 
1,000  candles;  and  for  it,  and  for  the  steam-engine 
that  works  it,  our  members  are  indebted  to  the  liberal- 
ity of  Dr.  William  Siemens,  who  in  the  most  generous 
manner  has  presented  the  machine  to  this  Institution. 
After  an  exhaustive  trial  at  the  South  Foreland,  ma- 
chines on  the  principle  of  Siemens,  but  of  far  greater 
power  than  this  one,  have  been  recently  chosen  by  the 
Elder  Brethren  of  the  Trinity  House  for  the  two  light- 
houses at  the  Lizard  Point. 

Our  most  intense  lights,  including  the  six-wick 
lamp,  the  Wigham  gas-light,  and  the  electric  light, 
being  intended  to  aid  the  mariner  in  heavy  weather, 
may  be  regarded,  in  a  certain  sense,  as  fog-signals. 
But  fog,  when  thick,  is  intractable  to  light.  The  sun 
cannot  penetrate  it,  much  less  any  terrestrial  source  of 
illumination.  Hence  the  necessity  of  employing  sound- 
signals  in  dense  fogs.  Bells,  gongs,  horns,  whistles, 


256  FRAGMENTS    OF    SCIENCE. 

guns,  and  syrens  have  been  used  for  this  purpose;  but 
it  is  mainly,  if  not  wholly,  with  explosive  signals  that 
we  have  now  to  deal.  The  gun  has  been  employed 
with  useful  effect  at  the  North  Stack,  near  Holyhead, 
on  the  Kish  Bank  near  Dublin,  at  Lundy  Island,  and 
at  other  points  on  our  coasts.  During  the  long,  labori- 
ous, and  I  venture  to  think  memorable  series  of  ob- 
servations conducted  under  the  auspices  of  the  Elder 
Brethren  of  the  Trinity  House  at  the  South  Foreland 
in  1872  and  1873,  it  was  proved  that  a  short  5|-inch 
howitzer,  firing  3  Ibs.  of  powder,  yielded  a  louder  re- 
port than  a  long  18-pounder  firing  the  same  charge. 
Here  was  a  hint  to  be  acted  on  by  the  Elder  Brethren. 
The  effectiveness  of  the  sound  depended  on  the  shape 
of  the  gun,  and  as  it  could  not  be  assumed  that  in  the 
howitzer  we  had  hit  accidentally  upon  the  best  possible 
shape,  arrangements  were  made  with  the  War  Office 
for  the  construction  of  a  gun  specially  calculated  to 
produce  the  loudest  sound  attainable  from  the  com- 
bustion of  3  Ibs.  of  powder.  To  prevent  the  unneces- 
sary landward  waste  of  the  sound,  the  gun  was  fur- 
nished with  a  parabolic  muzzle,  intended  to  project  the 
sound  over  the  sea,  where  it  was  most  needed.  The 
construction  of  this  gun  was  based  on  a  searching  series 
of  experiments  executed  at  Woolwich  with  small  mod- 
els, provided  with  muzzles  of  various  kinds.  A  draw- 
ing of  the  gun  is  annexed  (p.  257).  It  was  constructed 
on  the  principle  of  the  revolver,  its  various  chambers 
being  loaded  and  brought  in  rapid  succession  into  the 
firing  position.  The  performance  of  the  gun  proved 
the  correctness  of  the  principles  on  which  its  construc- 
tion was  based. 

An  incidental  point  of  some  interest  was  decided  by 
the  earliest  Woolwich  experiments.  It  had  been  a 
widely  spread  opinion  among  artillerists,  that  a  bronze 


RECENT    EXPERIMENTS   ON    FOG-SIGNALS.     25? 

gun  produces  a  specially  loud  report.  I  doubted  from 
the  outset  whether  this  would  help  us;  and  in  a  letter 
dated  22nd  April,  1874,  I  ventured  to  express  myself 
thus: — '  The  report  of  a  gun,  as  affecting  an  observer 
close  at  hand,  is  made  up  of  two  factors — the  sound 
due  to  the  shock  of  the  air  by  the  violently  expanding 
gas,  and  the  sound  derived  from  the  vibrations  of  the 
gun,  which,  to  some  extent,  rings  like  a  bell.  This 


Breech-loading  Foz-Bijrnal  Gun,  with  Bell  Mouth,*  proposed  by 
Major  Maitland,  K.  A.,  Assistant  Superintendent 


latter,  I  apprehend,  will  disappear  at  considerable  dis- 
tances.' The  result  of  subsequent  trial,  as  reported  by 
General  Campbell,  is,  '  that  the  sonorous  qualities  of 
bronze  are  greatly  superior  to  those  of  cast  iron  at  short 
distances,  but  that  the  advantage  lies  with  the  baser 
metal  at  long  ranges.'  f 

*  The  carriage  of  this  gun  has  been  modified  in  construction 
since  this  drawing  was  made. 

f  General  Campbell  assigns  a  tnie  cause  for  this  difference. 
The  ring  of  the  bronze  gun  represents  so  much  energy  withdrawn 


258  FRAGMENTS    OF    SCIENCE. 

Coincident  with  these  trials  of  guns  at  Woolwich, 
gun-cotton  was  thought  of  as  a  probably  effective 
sound-producer.  From  the  first,  indeed,  theoretic  con- 
siderations caused  me  to  fix  my  attention  persistently 
on  this  substance;  for  the  remarkable  experiments  of 
Mr.  Abel,  whereby  its  rapidity  of  combustion  and  vio- 
lently explosive  energy  are  demonstrated,  seemed  to 
single  it  out  as  a  substance  eminently  calculated  to  ful- 
fil the  conditions  necessary  to  the  production  of  an 
intense  wave  of  sound.  What  those  conditions  are  we 
shall  now  more  particularly  enquire,  calling  to  our 
aid  a  brief  but  very  remarkable  paper,  published  by 
Professor  Stokes  in  the  '  Philosophical  Magazine '  for 
1868. 

The  explosive  force  of  gunpowder  is  known  to  de- 
pend on  the  sudden  conversion  of  a  solid  body  into  an 
intensely  heated  gas.  Now  the  work  which  the  artil- 
lerist requires  the  expanding  gas  to  perform  is  the  dis- 
placement of  the  projectile,  besides  which  it  has  to  dis- 
place the  air  in  front  of  the  projectile,  which  is  backed 
by  the-  whole  pressure  of  the  atmosphere.  Such,  how- 
ever, is  not  the  work  that  we  want  our  gunpowder  to 
perform.  We  wish  to  transmute  its  energy  not  into  the 
mere  mechanical  translation  of  either  shot  or  air,  but 
into  vibratory  motion.  We  want  pulses  to  be  formed 
which  shall  propagate  themselves  to  vast  distances 
through  the  atmosphere,  and  this  requires  a  certain 
choice  and  management  of  the  explosive  material. 

A  sound-wave  consists  essentially  of  two  parts — a 
condensation  and  a  rarefaction.  Now  air  is  a  very 
mobile  fluid,  and  if  the  shock  imparted  to  it  lacks  due 
promptness,  the  wave  is  not  produced.  Consider  the 

from  the  explosive  force  of  the  gunpowder.  Further  experiments 
would,  however,  be  needed  to  place  the  superiority  of  the  cast- 
iron  gun  at  a  distance  beyond  question. 


RECENT    EXPERIMENTS    ON    FOG-SIGNALS.      259 

case  of  a  common  clock  pendulum,  which  oscillates  to 
and  fro,  and  which  might  be  expected  to  generate  cor- 
responding pulses  in  the  air.  When,  for  example,  the 
bob  moves  to  the  right,  the  air  to  the  right  of  it  might 
be  supposed  to  be  condensed,  while  a  partial  vacuum 
might  be  supposed  to  follow  the  bob.  As  a  matter  of 
fact,  we  have  nothing  of  the  kind.  The  air  particles  in 
front  of  the  bob  retreat  so  rapidly,  and  those  behind  it 
close  so  rapidly  in,  that  no  sound-pulse  is  formed.  The 
mobility  of  hydrogen,  moreover,  being  far  greater  than 
that  of  air,  a  prompter  action  is  essential  to  the  forma- 
tion of  sonorous  waves  in  hydrogen  than  in  air.  It  is 
to  this  rapid  power  of  readjustment,  this  refusal,  so  to 
speak,  to  allow  its  atoms  to  be  crowded  together  or  to 
be  drawn  apart,  that  Professor  Stokes,  with  admirable 
penetration,  refers  the  damping  power,  first  described 
by  Sir  John  Leslie,  of  hydrogen  upon  sound. 

A  tuning-fork  which  executes  256  complete  vibra- 
tions in  a  second,  if  struck  gently  on  a  pad  and  held  in 
free  air,  emits  a  scarcely  audible  note.  It  behaves  to 
some  extent  like  the  pendulum  bob  just  referred  to. 
This  feebleness  is  due  to  the  prompt  *  reciprocating 
flow*  of  the  air  between  the  incipient  condensations 
and  rarefactions,  whereby  the  formation  of  sound- 
pulses  is  forestalled.  Stokes,  however,  has  taught  us 
that  this  flow  may  be  intercepted  by  placing  the  edge 
of  a  card  in  close  proximity  to  one  of  the  corners  of  the 
fork.  An  immediate  augmentation  of  the  sound  of  the 
fork  is  the  consequence. 

The  more  rapid  the  shock  imparted  to  the  air,  the 
greater  is  the  fractional  part  of  the  energy  of  the  shock 
converted  into  wave  motion.  And  as  different  kinds  of 
gunpowder  vary  considerably  in  their  rapidity  of  com- 
bustion, it  may  be  expected  that  they  will  also  vary  as 
producers  of  sound.  This  theoretic  inference  is  com- 


260  FRAGMENTS    OF    SCIE^TCE. 

pletely  verified  by  experiment.  In  a  series  of  prelimi- 
nary trials  conducted  at  Woolwich  on  the  4th  of  June, 
1875,  the  sound-producing  powers  of  four  different 
kinds  of  powder  were  determined.  In  the  order  of  the 
size  of  their  grains  they  bear  the  names  respectively  of 
Fine-grain  (F.  G.),  Large-grain  (L.  G.),  Eifle  Large- 
grain  (E.  L.  G.),  and  Pebble-powder  (P.).  (See  an- 
nexed figures.)  The  charge  in  each  case  amounted  to 

FIG.  7. 


F.  G.  L.  G.  R.  L.  G. 

4£  Ibs.;  four  24-lb.  howitzers  being  employed  to  fire  the 
respective  charges.  There  were  eleven  observers,  all  of 
whom,  without  a  single  dissentient,  pronounced  the 
sound  of  the  fine-grain  powder  loudest  of  all.  In  the 
opinion  of  seven  of  the  eleven  the  large-grain  powder 
came  next;  seven  also  of  the  eleven  placed  the  rifle 
large-grain  third  on  the  list;  while  they  were  again 
unanimous  in  pronouncing  the  pebble-powder  the 
worst  sound-producer.  These  differences  are  entirely 
due  to  differences  in  the  rapidity  of  combustion.  All 
who  have  witnessed  the  performance  of  the  80-ton  gun 
must  have  been  surprised  at  the  mildness  of  its  thun- 
der. To  avoid  the  strain  resulting  from  quick  combus- 
tion, the  powder  employed  is  composed  of  lumps  far 
larger  than  those  of  the  pebble-powder  above  referred 
to.  In  the  long  tube  of  the  gun  these  lumps  of  solid 
matter  gradually  resolve  themselves  into  gas,  which  on 
issuing  from  the  muzzle  imparts  a  kind  of  push  to  the 


RECENT    EXPERIMENTS    ON    FOG-SIGNALS.     201 

air,  instead  of  the  sharp  shock  necessary  to  form  the 
condensation  of  an  intensely  sonorous  wave. 

These  are  some  of  the  physical  reasons  why  gun- 
cotton  might  be  regarded  as  a  promising  fog-signal. 
Firing  it  as  we  have  been  taught  to  do  by  Mr.  Abel, 
its  explosion  is  more  rapid  than  that  of  gunpowder. 
In  its  case  the  air  particles,  alert  as  they  are,  will  not, 
it  might  be  presumed,  be  able  to  slip  from  condensation 
to  rarefaction  with  a  rapidity  sufficient  to  forestall  the 
formation  of  the  wave.  On  a  priori  grounds  then,  we 
are  entitled  to  infer  the  effectiveness  of  gun-cotton, 
while  in  a  great  number  of  comparative  experiments, 
stretching  from  1874  to  the  present  time,  this  infer- 
ence has  been  verified  in  the  most  conclusive  manner. 

As  regards  explosive  material,  and  zealous  and  ac- 
complished help  in  the  use  of  it,  the  resources  of 
Woolwich  Arsenal  have  been  freely  placed  at  the  dis- 
posal of  the  Elder  Brethren.  General  Campbell,  Gen- 
eral Younghusband,  Colonel  Fraser,  Colonel  Maitland, 
and  other  officers,  have  taken  an  active  personal  part  in 
the  investigation,  and  in  most  cases  have  incurred  the 
labour  of  reducing  and  reporting  on  the  observations. 
Guns  of  various  forms  and  sizes  have  been  invoked  for 
gunpowder,  while  gun-cotton  has  been  fired  in  free  air 
and  in  the  foci  of  parabolic  reflectors. 

On  the  22nd  of  February,  1875,  a  number  of  small 
guns,  cast  specially  for  the  purpose — some  with  plain, 
some  with  conical,  and  some  with  parabolic  muzzles — 
firing  4  oz.  of  fine  grain  powder,  were  pitted  against 
4  oz.  of  gun-cotton  detonated  both  in  the  open,  and  in 
the  focus  of  a  parabolic  reflector.*  The  sound  pro- 
duced by  the  gun-cotton,  reinforced  by  the  reflector, 
was  unanimously  pronounced  loudest  of  all.  With 

*  For  charges  of  this  weight  the  reflector  is  of  moderate  size, 
and  rauy  be  employed  without  fear  of  fracture. 


262  FRAGMENTS    OF    SCIENCE. 

equal  unanimity,  the  gun-cotton  detonated  in  free  air 
was  placed  second  in  intensity.  Though  the  same 
charge  was  used  throughout,  the  guns  differed  notably 
among  themselves,  but  none  of  them  came  up  to  the 
gun-cotton,  either  with  or  without  the  reflector.  A 
second  series,  observed  from  a  different  distance  on  the 
same  day,  confirmed  to  the  letter  the  foregoing  result. 

As  a  practical  point,  however,  the  comparative  cost 
of  gun-cotton  and  gunpowder  has  to  be  taken  into 
account,  though  considerations  of  cost  ought  not  to  be 
stretched  too  far  in  cases  involving  the  safety  of  human 
life.  In  the  earlier  jexperiments,  where  quantities  of 
equal  price  were  pitted  against  each  other,  the  results 
were  somewhat  fluctuating.  Indeed,  the  perfect  ma- 
nipulation of  the  gun-cotton  required  some  prelimi- 
nary discipline — promptness,  certainty,  and  effective- 
ness of  firing,  augmenting  as  experience  increased. 
As  1  Ib.  of  gun-cotton  costs  as  much  as  3  Ibs.  of  gun- 
powder, these  quantities  were  compared  together  on 
the  22nd  of  February.  The  guns  employed  to  dis- 
charge the  gunpowder  were  a  12-lb.  brass  howitzer,  a 
24-lb.  cast-iron  howitzer,  and  the  long  18-pounder  em- 
ployed at  the  South  Foreland.  The  result  was,  that 
the  24-lb.  howitzer,  firing  3  Ibs.  of  gunpowder,  had  a 
slight  advantage  over  1  Ib.  of  gun-cotton  detonated  in 
the  open;  while  the  12-lb.  howitzer  and  the  18-pounder 
were  both  beaten  by  the  gun-cotton.  On  the  2nd  of 
May,  on  the  other  hand,  the  gun-cotton  is  reported  as 
having  been  beaten  by  all  the  guns. 

Meanwhile,  the  parabolic-muzzle  gun,  expressly  in- 
tended for  fog-signalling,  was  pushed  rapidly  forward, 
and  on  March  22  and  23,  1876,  its  power  was  tested  at 
Shoeburyness.  Pitted  against  it  were  a  16-pounder,  a 
5£-ineh  howitzer,  1^  Ib.  of  gun-cotton  detonated  in  the 
focus  of  a  reflector  (see  annexed  figure),  and  1|  Ib.  of 


RECENT   EXPERIMENTS   ON   FOG-SIGNALS.     263 

gun-cotton  detonated  in  free  air.  On  this  occasion 
nineteen  different  series  of  experiments  were  made, 
when  the  new  experimental  gun,  firing  a  3-lb.  charge, 
demonstrated  its  superiority  over  all  guns  previously 
employed  to  fire  the  same  charge.  As  regards  the  com- 

FJO.  8. 


Gun-cotton  Slab  (H  lb.)  Detonated  in  the  Focus  of  a  Cast-iron 
Keflcctor. 


parative  merits  of  the  gun-cotton  fired  in  the  open, 
and  the  gun-powder  fired  from  the  new  gun,  the  mean 
values  of  their  sounds  were  the  same.  Fired  in  the 
focus  of  the  reflector,  the  gun-cotton  clearly  dominated 
over  all  the  other  sound-producers.* 

The  whole  of  the  observations  here  referred  to  were 
embraced  by  an  angle  of  about  70°,  of  which  50°  lay  on 

*  The  reflector  was  frnctured  by  the  explosion,  but  it  did 
good  service  afterwards. 
18 


264  FRAGMENTS    OF    SCIENCE. 

the  one  side  and  20°  on  the  other  side  of  the  line  of 
fire.  The  shots  were  heard  by  eleven  observers  on 
board  the  '  Galatea,'  which  took  up  positions  varying 
from  2  miles  to  13|  miles  from  the  firing-point.  In 
all  these  observations,  the  reinforcing  action  of  the  re- 
flector, and  of  the  parabolic  muzzle  of  the  gun,  came 
into  play.  But  the  reinforcement  of  the  sound  in  one 
direction  implies  its  withdrawal  from  some  other  di- 
rection, and  accordingly  it  was  found  that  at  a  distance 
of  5£  miles  from  the  firing-point,  and  on  a  line  includ- 
ing nearly  an  angle  of  90°  with  the  line  of  fire,  the 
gun-cotton  in  the  open  beat  the  new  gun;  while  be- 
hind the  station,  at  distances  of  8^  miles  and  13|  miles 
respectively,  the  gun-cotton  in  the  open  beat  both  the 
gun  and  the  gun-cotton  in  the  reflector.  This  result 
is  rendered  more  important  by  the  fact  that  the  sound 
reached  the  Mucking  Light,  a  distance  of  13^  miles, 
against  a  light  wind  which  was  blowing  at  the  time. 
Most,  if  not  all,  of  our  ordinary  sound-producers 
send  forth  waves  which  are  not  of  uniform  intensity 
throughout.  A  trumpet  is  loudest  in  the  direction  of 
its  axis.  The  same  is  true  of  a  gun.  A  bell,  with  its 
mouth  pointed  upwards  or  downwards,  sends  forth 
waves  which  are  far  denser  in  the  horizontal  plane 
passing  through  the  bell  than  at  an  angular  distance 
of  90°  from  that  plane.  The  oldest  bellhangers  must 
have  been  aware  of  the  fact  that  the  sides  of  the  bell, 
and  not  its  mouth,  emitted  the  strongest  sound,  their 
practice  being  probably  determined  by  this  knowledge. 
Our  slabs  of  gun-cotton  also  emit  waves  of  different 
densities  in  different  parts.  It  has  occurred  in  the  ex- 
periments at  Shoeburyness  that  when  the  broad  side 
of  a  slab  was  turned  towards  the  suspending  wire  of  a 
second  slab  six  feet  distant,  the  wire  was  cut  by  the 
explosion,  while  when  the  edge  of  the  slab  was  turned 


RECENT    EXPERIMENTS    ON    FOG-SIGNALS.     265 

to  the  wire  this  never  occurred.  To  the  circumstance 
that  the  broadsides  of  the  slabs  faced  the  sea  is  prob- 
ably to  be  ascribed  the  remarkable  fact  observed  on 
March  23,  that  in  two  directions,  not  far  removed  from 
the  line  of  fire,  the  gun-cotton  detonated  in  the  open 
had  a  slight  advantage  over  the  new  gun. 

Theoretic  considerations  rendered  it  probable  that 
the  shape  and  size  of  the  exploding  mass  would  affect 
the  constitution  of  the  wave  of  sound.  I  did  not  think 
large  rectangular  slabs  the  most  favourable  shape,  and 
accordingly  proposed  cutting  a  large  slab  into  frag- 
ments of  different  sizes,  and  pitting  them  against  each 
other.  The  differences  between  the  sounds  were  by  no 
means  so  great  as  the  differences  in  the  quantities  of 
explosive  material  might  lead  one  to  expect.  The 
mean  values  of  eighteen  series  of  observations  made  on 
board  the  '  Galatea,'  at  distances  varying  from  If  mile 
to  4.8  miles,  were  as  follows: — 

Weights      .        .    4  oz.        6  oz.        9  oz.        12  oz. 
Value  of  sound  .    3.12         3.34         4.0  4.03 

These  charges  were  cut  from  a  slab  of  dry  gun- 
cotton  about  If  inch  thick:  they  were  squares  and  rec- 
tangles of  the  following  dimensions: — 4  oz.,  2  inches 
by  2  inches;  6  oz.,  2  inches  by  3  inches;  9  oz.,  3  inches 
by  3  inches;  12  oz.,  2  inches  by  6  inches. 

The  numbers  under  the  respective  weights  express 
the  recorded  value  of  the  sounds.  They  must  be  sim- 
ply taken  as  a  ready  means  of  expressing  the  approxi- 
mate relative  intensity  of  the  sounds  as  estimated  by 
the  ear.  When  we  find  a  9-oz.  charge  marked  4,  and  a 
12-oz.  charge  marked  4.03,  the  two  sounds  may  be  re- 
garded as  practically  equal  in  intensity,  thus  proving 
that  an  addition  of  30  per  cent,  in  the  larger  charges 
produces  no  sensible  difference  in  the  sound.  Were 


266  FRAGMENTS    OF    SCIENCE. 

the  sounds  estimated  by  some  physical  means,  instead 
of  by  the  ear,  the  values  of  the  sounds  at  the  distances 
recorded  would  not,  in  my  opinion,  show  a  greater 
advance  with  the  increase  of  material  than  that  indi- 
cated by  the  foregoing  numbers.  Subsequent  experi- 
ments rendered  still  more  certain  the  effectiveness, 
as  well  as  the  economy,  of  the  smaller  charges  of  gun- 
cotton. 

It  is  an  obvious  corollary  from  the  foregoing  ex- 
periments that  on  our  '  nesses '  and  promontories, 
where  the  land  is  clasped  on  both  sides  for  a  consider- 
able distance  by  the  sea — where,  therefore,  the  sound 
has  to  propagate  itself  rearward  as  well  as  forward — 
the  use  of  the  parabolic  gun,  or  of  the  parabolic  re- 
flector, might  be  a  disadvantage  rather  than  an  advan- 
tage. Here  gun-cotton,  exploded  in  the  open,  forms 
the  most  appropriate  source  of  sound.  This  remark 
is  especially  applicable  to  such  lightships  as  are  in- 
tended to  spread  the  sound  all  round  them  as  from  cen- 
tral foci.  As  a  signal  in  rock  lighthouses,  where  nei- 
ther syren,  steam-whistle,  nor  gun  could  be  mounted; 
and  as  a  handy  fleet-signal,  dispensing  with  the  lumber 
of  special  signal-guns,  the  gun-cotton  will  prove  in- 
valuable. But  in  most  of  these  cases  we  have  the 
drawback  that  local  damage  may  be  done  by  the  ex- 
plosion. The  lantern  of  the  rock  lighthouse  might 
suffer  from  concussion  near  at  hand,  and  though  me- 
chanical arrangements  might  be  devised,  both  in  the 
case  of  the  lighthouse  and  of  the  ship's  deck,  to  place 
the  firing-point  of  the  gun-cotton  at  a  safe  distance, 
no  such  arrangement  could  compete,  as  regards  sim- 
plicity and  effectiveness,  with  the  expedient  of  a  gun- 
cotton  rocket.  Had  such  a  means  of  signalling  existed 
at  the  Bishop's  Rock  lighthouse,  the  ill-fated  'Schiller' 
might  have  been  warned  of  her  approach  to  danger  ten, 


RECENT    EXPERIMENTS    ON    FOG-SIGNALS.     267 

or  it  may  be  twenty,  miles  before  she  reached  the  rock 
which  wrecked  her.  Had  the  fleet  possessed  such  a 
signal,  instead  of  the  ubiquitous  but  ineffectual  whis- 
tle, the  '  Iron  Duke '  and  *  Vanguard '  need  never  have 
come  into  collision. 

It  was  the  necessity  of  providing  a  suitable  signal 
for  rock  lighthouses,  and  of  clearing  obstacles  which 
cast  an  acoustic  shadow,  that  suggested  the  idea  of  the 
gun-cotton  rocket  to  Sir  Richard  Collinson,  Deputy 
Master  of  the  Trinity  House.  His  idea  was  to  place  a 
disk  or  short  cylinder  of  gun-cotton  in  the  head  of  a 
rocket,  the  ascensional  force  of  which  should  be  em- 
ployed to  carry  the  disk  to  an  elevation  of  1,000  feet  or 
thereabouts,  where  by  the  ignition  of  a  fuse  associated 
with  a  detonator,  the  gun-cotton  should  be  fired,  send- 
ing its  sound  in  all  directions  vertically  and  obliquely 
down  upon  earth  and  sea.  The  first  attempt  to  realise 
this  idea  was  made  on  July  18,  1876,  at  the  firework 
manufactory  of  the  Messrs.  Brock,  at  Nunhead.  Eight 
rockets  were  then  fired,  four  being  charged  with  5  oz. 
and  four  with  7£  oz.  of  gun-cotton.  They  ascended  to 
a  great  height,  and  exploded  with  a  very  loud  report  in 
the  air.  On  July  27,  the  rockets  were  tried  at  Shoe- 
buryness.  The  most  noteworthy  result  on  this  occa- 
sion was  the  hearing  of  the  sounds  at  the  Mouse  Light- 
house, 8£  miles  E.  by  S.,  and  at  the  Chapman  Light- 
house, 8£  miles  W.  .by  N.;  that  is  to  say,  at  opposite 
sides  of  the  firing-point.  It  is  worthy  of  remark  that, 
in  the  case  of  the  Chapman  Lighthouse,  land  and  trees 
intervened  between  the  firing-point  and  the  place  of 
observation.  '  This,'  as  General  Younghusband  justly 
remarked  at  the  time,  *  may  prove  to  be  a  valuable  con- 
sideration if  it  should  be  found  necessary  to  place  a 
signal  station  in  a  position  whence  the  sea  could  not 
be  freely  observed.'  Indeed,  the  clearing  of  such  ob- 


268  FRAGMENTS    OF    SCIENCE. 

stacks  was  one  of  the  objects  which  the  inventor  of  the 
rocket  had  in  view. 

With  reference  to  the  action  of  the  wind,  it  Avas 
thought  desirable  to  compare  the  range  of  explosions 
produced  near  the  surface  of  the  earth  with  others 
produced  at  the  elevation  attainable  by  the  gun-cotton 
rockets.  Wind  and  weather,  however,  are  not  at  our 
command;  and  hence  one  of  the  objects  of  a  series  of 
experiments  conducted  on  December  13,  1876,  was  not 
fulfilled.  It  is  worthy,  however,  of  note  that  on  this 
day,  with  smooth  water  and  a  calm  atmosphere,  the 
rockets  were  distinctly  heard  at  a  distance  of  11.2  miles 
from  the  firing-point.  The  quantity  of  gun-cotton 
employed  was  7^  oz.  On  Thursday,  March  8,  1877, 
these  comparative  experiments  of  firing  at  high  and 
low  elevations  were  pushed  still  further.  The  gun- 
cotton  near  the  ground  consisted  of  ^-lb.  disks,  sus- 
pended from  a  horizontal  iron  bar  about  4£  feet  above 
the  ground.  The  rockets  carried  the  same  quantity 
of  gun-cotton  in  their  heads,  and  the  height  to  which 
they  attained,  as  determined  by  a  theodolite,  was  from 
800  to  900  feet.  The  day  was  cold,  with  occasional 
squalls  of  snow  and  hail,  the  direction  of  the  sound 
being  at  right  angles  to  that  of  the  wind.  Five  series 
of  observations  were  made  on  board  the  '  Vestal,'  at 
distances  varying  from  3  to  6  miles.  The  mean  value 
of  the  explosions  in  the  air  exceeded  that  of  the  ex- 
plosions near  the  ground  by  a  small  but  sensible  quan- 
tity. At  Windmill  Hill,  Gravesend,  however,  which 
was  nearly  to  leeward,  and  5^  miles  from  the  firing- 
point,  in  nineteen  cases  out  of  twenty-four  the  disk 
fired  near  the  ground  was  loudest;  while  in  the  re- 
maining five  the  rocket  had  the  advantage. 

Towards  the  close  of  the  day  the  atmosphere  be- 
came very  serene.  A  few  distant  cumuli  sailed  near 


RECENT    EXPERIMENTS    ON    FOG-SIGNALS.     209 

the  horizon,  but  the  zenith  and  a  vast  angular  space  all 
round  it  were  absolutely  free  from  cloud.  From  the 
deck  of  the  '  Galatea '  a  rocket  was  discharged,  which 
reached  a  great  elevation,  and  exploded  with  a  loud 
report.  Following  this  solid  nucleus  of  sound  was  a 
continuous  train  of  echoes,  which  retreated  to  a  con- 
tinually greater  distance,  dying  gradually  off  into  si- 
lence after  seven  seconds'  duration.  These  echoes 
were  of  the  same  character  as  those  so  frequently  no- 
ticed at  the  South  Foreland  in  1872-73,  and  called  by 
me  '  aerial  echoes/ 

On  the  23rd  of  March  the  experiments  were  re- 
sumed, the  most  noteworthy  results  of  that  day's  ob- 
servations being  that  the  sounds  were  heard  at  Tilling- 
ham,  10  miles  to  the  N.E.;  at  West  Mersea,  lof  miles 
to  the  N.E.  by  E.;  at  Brightlingsea,  17£  miles  to  the 
N.E.;  and  at  Clacton  Wash,  20£  miles  to  the  N.E. 
by  ^  E.  The  wind  was  blowing  at  the  time  from  the 
S.E.  Some  of  these  sounds  were  produced  by  rockets, 
some  by  a  24-lb.  howitzer,  and  some  by  an  8-inch 
Maroon. 

In  December,  1876,  Mr.  Gardiner,  the  managing 
director  of  the  Cotton-powder  Company,  had  proposed 
a  trial  of  this  material  against  the  gun-cotton.  The 
density  of  the  cotton  he  urged  was  only  1.03,  while 
that  of  the  powder  was  1.70.  A  greater  quantity  of 
explosive  material  being  thus  compressed  into  the  same 
volume,  Mr.  Gardiner  thought  that  a  greater  sonorous 
effect  must  be  produced  by  the  powder.  At  the  in- 
stance of  Mr.  Mackie,  who  had  previously  gone  very 
thoroughly  into  the  subject,  a  Committee  of  the  Elder 
Brethren  visited  the  cotton-powder  manufactory,  on 
the  banks  of  the  Swale,  near  Faversham,  on  the  16th 
of  June,  1877.  The  weights  of  cotton-powder  em- 
ployed were  2  oz.,  8  oz.,  1  lb.,  and  2  Ibs.,  in  the  form  of 


270  FRAGMENTS    OF    SCIENCE. 

rockets  and  of  signals  fired  a  few  feet  above  the  ground. 
The  experiments  throughout  were  arranged  and  con- 
ducted by  Mr.  Mackie.  Our  desire  on  this  occasidn  was 
to  get  as  near  to  windward  as  possible,  but  the  Swale 
and  other  obstacles  limited  our  distance  to  1^  mile. 
We  stood  here  E.S.E.  from  the  firing-point  while  the 
wind  blew  fresh  from  the  N.E. 

The  cotton-powder  yielded  a  very  effective  report. 
The  rockets  in  general  had  a  slight  advantage  over  the 
same  quantities  of  material  fired  near  the  ground.  The 
loudness  of  the  sound  was  by  no  means  proportional 
to  the  quantity  of  the  material  exploded,  8  oz.  yielding 
very  nearly  as  loud  a  report  as  1  Ib.  The  *  aerial 
echoes/  which  invariably  followed  the  explosion  of  the 
rockets,  were  loud  and  long-continued. 

On  the  17th  of  October,  1877,  another  series  of  ex- 
periments with  howitzers  and  rockets  was  carried  out 
at  Shoeburyness.  The  charge  of  the  howitzer  was  3 
Ibs.  of  L.  G.  powder.  The  charges  of  the  rockets  were 
12  oz.,  8  oz.,  4  oz.,  and  2  oz.  of  gun-cotton  respectively. 
The  gun  and  the  four  rockets  constituted  a  series,  and 
eight  series  were  fired  during  the  afternoon  of  the  17th. 
The  observations  were  made  from  the  '  Vestal '  and  the 
'  Galatea/  positions  being  successively  assumed  which 
permitted  the  sound  to  reach  the  observers  with  the 
wind,  against  the  wind,  and  across  the  wind.  The  dis- 
tance of  the  '  Galatea '  varied  from  3  to  7  miles,  that  of 
the  '  Vestal/  which  was  more  restricted  in  her  move- 
ments, being  2  to  3  miles.  Briefly  summed  up,  the 
result  is  that  the  howitzer,  firing  a  3-lb.  charge,  which 
it  will  be  remembered  was  our  best  gun  at  the  South 
Foreland,  was  beaten  by  the  12-oz.  rocket,  by  the  8-oz. 
rocket,  and  by  the  4-oz.  rocket.  The  2-oz.  rocket 
alone  fell  behind  the  howitzer. 

It  is  worth  while  recording  the  distances  at  which 


RECENT   EXPERIMENTS   ON    FOG-SIGNALS.     271 

some  of  the  sounds  were  heard  on  the  day  now  re- 
ferred to:  — 

24  out  of  40  sounds  heard. 


1.  Leigh      .      . 

6±  miles  W.N.W.       24 

2.  Girdler  Light- 

vessel    . 

12     " 

S.E.  by  E.     5 

3.  Reculvers 

17i  « 

S.E.  byS.    18 

4  St.  Nicholas  . 

20     " 

S.E.     .          3 

5.  Epple  Bay     . 

22     " 

S.E.  by  E.  19 

G.  Westgate 

23     " 

S.E.  by  E.     9 

7.  Kingsgate     . 

25     " 

S.E.  by  E.     8 

The  day  was  cloudy,  with  occasional  showers  of 
drizzling  rain;  the  wind  about  N.W.  by  N.  all  day;  at 
times  squally,  rising  to  a  force  of  6  or  7  and  sometimes 
dropping  to  a  force  of  2  or  3.  The  station  at  Leigh 
excepted,  all  these  places  were  to  leeward  of  Shoebury- 
ness.  At  four  other  stations  to  leeward,  varying  in 
distance  from  15£  to  24£  miles,  nothing  was  heard, 
while  at  eleven  stations  to  windward,  varying  from  8  to 
26  miles,  the  sounds  were  also  inaudible.  It  was  found, 
indeed,  that  the  sounds  proceeding  directly  against  the 
wind  did  not  penetrate  much  beyond  3  miles. 

On  the  following  day,  viz.  the  18th  October,  we 
proceeded  to  Dungeness  with  the  view  of  making  a 
series  of  strict  comparative  experiments  with  gun- 
cotton  and  cotton-powder.  Rockets  containing  8  oz., 
4  oz.,  and  2  oz.  of  gun-cotton  had  been  prepared  at  the 
Royal  Arsenal;  while  others,  containing  similar  quan- 
tities of  cotton-powder,  had  been  supplied  by  the  Cot- 
ton-powder Company  at  Faversham.  With  these  were 
compared  the  ordinary  18-pounder  gun,  which  hap- 
pened to  be  mounted  at  Dungeness,  firing  the  usual 
charge  of  3  Ibs.  of  powder,  and  a  syren. 

From  these  experiments  it  appeared  that  the  gun- 
cotton  and  cotton-powder  were  practically  equal  as 
producers  of  sound. 


272  FRAGMENTS    OF    SCIENCE. 

The  effectiveness  of  small  charges  was  illustrated  in 
a  very  striking  manner,  only  a  single  unit  separating 
the  numerical  value  of  the  8-oz.  rocket  from  that  of  the 
2-oz.  rocket.  The  former  was  recorded  as  6.9  and  the 
latter  as  5.9,  the  value  of  the  4-oz.  rocket  being  inter- 
mediate between  them.  These  results  were  recorded 
by  a  number  of  very  practised  observers  on  board  the 
'  Galatea/  They  were  completely  borne  out  by  the 
observations  of  the  Coastguard,  who  marked  the  value 
of  the  8-oz.  rocket  6.1,  and  that  of  the  2-oz.  rocket  5.2. 
The  18-pounder  gun  fell  far  behind  all  the  rockets,  a 
result,  possibly,  to  be  in  part  ascribed  to  the  imperfec- 
tion of  the  powder.  The  performance  of  the  syren  was, 
on  the  whole,  less  satisfactory  than  that  of  the  rocket. 
The  instrument  was  worked,  not  by  steam  of  70  Ibs. 
pressure,  as  at  the  South  Foreland,  but  by  compressed 
air,  beginning  with  40  Ibs.  and  ending  with  30  Ibs. 
pressure.  The  trumpet  was  pointed  to  windward,  and 
in  the  axis  of  the  instrument  the  sound  was  about  as 
effective  as  that  of  the  8-oz.  rocket.  But  in  a  direction 
at  right  angles  to  the  axis,  and  still  more  in  the  rear  of 
this  direction,  the  syren  fell  very  sensibly  behind  even 
the  2-oz.  rocket. 

These  are  the  principal  comparative  trials  made  be- 
tween the  gun-cotton  rocket  and  other  fog-signals;  but 
they  are  not  the  only  ones.  On  the  2nd  of  August, 
1877,  for  example,  experiments  were  made  at  Lundy 
Island  with  the  following  results.  At  2  miles  distant 
from  the  firing-point,  with  land  intervening,  the  18- 
pounder,  firing  a  3-lb.  charge,  was  quite  unheard. 
Both  the  4-oz.  rocket  and  the  8-oz.  rocket,  however, 
reached  an  elevation  which  commanded  the  acoustic 
shadow,  and  yielded  loud  reports.  When  both  were  in 
view  the  rockets  were  still  superior  to  the  gun.  On 
the  6th  of  August,  at  St.  Ann's,  the  4-oz.  and  8-oz. 


RECENT    EXPERIMENTS   ON    FOG-SIGNALS.     273 

rockets  proved  superior  to  the  syren.  On  the  Shambles 
Light-vessel,  when  a  pressure  of  13  Ibs.  was  employed 
to  sound  the  syren,  the  rockets  proved  greatly  superior 
to  that  instrument.  Proceeding  along  the  sea  margin 
at  Flamboro'  Head,  Mr.  Edwards  states  that  at  a  dis- 
tance of  1£  mile,  with  the  18-pounder  previously  used 
as  a  fog-signal  hidden  behind  the  cliffs,  its  report  was 
quite  unheard,  while  the  4-oz.  rocket,  rising  to  an  ele- 
vation which  brought  it  clearly  into  view,  yielded  a 
powerful  sound  in  the  face  of  an  opposing  wind. 

On  the  evening  of  February  9th,  1877,  a  remark- 
able series  of  experiments  were  made  by  Mr.  Prentice 
at  Stowmarket  with  the  gun-cotton  rocket.  From  the 
report  with  which  he  has  kindly  furnished  me  I  extract 
the  following  particulars.  The  first  column  in  the  an- 
nexed statement  contains  the  name  of  the  place  of  ob- 
servation, the  second  its  distance  from  the  firing-point, 
and  the  third  the  result  observed. 

Stoke  Hill,  Ipswich  .  10  miles  Rockets  clearly  seen  and  sounds 

distinctly  heard  53  seconds 
after  the  flash. 

Melton  .  .  .  .  15  "  Signals  distinctly  heard. 
Thought  at  first  that  sounds 
were  reverberated  from  the 
sea. 

Framlingham.  .  .  18  "  Signals  very  distinctly  heard, 
both  in  the  open  air  and  in  a 
closed  room.  Wind  in  favour 
of  sound. 

Stratford.  St.  Andrews  19  "  Reports  loud :  startled  pheas- 
ants in  a  cover  close  by. 

Tuddenham.  St.  Martin  10  "  Reports  very  loud ;  rolled  away 

like  thunder. 

Christ  Church  Park  .11  "  Report  arrived  a  little  more 

than  a  minute  after  flash. 

Nettlestead  Hall  .  .  6  "  Distinct  in  every  part,  of  ob- 
server's house.  Very  loud 
in  the  open  air. 


274  FKAGMENTS    OF    SCIENCE. 

Bildestone  .  .  .  6  "  Explosion  very  loud,  wind 
against  sound. 

Nacton 14  "  Reports  quite  distinct — mis- 
taken by  inhabitants  for 
claps  of  thunder. 

Aldboro' .  .  .  .  25  "  Rockets  seen  through  a  very 
ha/y  atmosphere;  a  rum- 
bling detonation  heard. 

Capel  Mills  .  .  .  11  "  Reports  heard  within  and  with- 
out the  observer's  house. 
Wind  opposed  to  sound. 

Lawford  ....  15|  "  Reports  distinct :  attributed  to 
distant  thunder. 

In  the  great  majority  of  these  cases,  the  direction 
of  the  sound  enclosed  a  large  angle  with  the  direction 
of  the  wind.  In  some  cases,  indeed,  the  two  directions 
were  at  right  angles  to  each  other.  It  is  needless  to 
dwell  for  a  moment  on  the  advantage  of  possessing  a 
signal  commanding  ranges  such  as  these. 

The  explosion  of  substances  in  the  air,  after  having 
been  carried  to  a  considerable  elevation  by  rockets,  is 
a  familiar  performance.  In  1873,  moreover,  the  Board 
of  Trade  proposed  a  light-and-sound  rocket  as  a  signal 
of  distress,  which  proposal  was  subsequently  realized, 
but  in  a  form  too  elaborate  and  expensive  for  practical 
use.  The  idea  of  a  gun-cotton  rocket  fit  for  signalling 
in  fogs  is,  I  believe,  wholly  due  to  Sir  Richard  Collin- 
son,  the  Deputy  Master  of  the  Trinity  House.  Thanks 
to  the  skilful  aid  given  by  the  authorities  of  Wool- 
wich, by  Mr.  Prentice,  and  Mr.  Brock,  that  idea  is  now 
an  accomplished  fact;  a  signal  of  great  power,  handi- 
ness,  and  economy,  being  thus  placed  at  the  service  of 
our  mariners.  Not  only  may  the  rocket  be  applied  in 
association  with  lighthouses  and  lightships,  but  in  the 
Navy  also  it  may  be  turned  to  important  account. 
Soon  after  the  loss  of  the  '  Vanguard '  I  ventured  to 
urge  upon  an  eminent  naval  officer  the  desirability 


RECENT    EXPERIMENTS   ON    FOG-SIGNALS.     275 

of  having  an  organized  code  of  fog-signals  for  the  fleet. 
He  shook  his  head  doubtingly,  and  referred  to  the  diffi- 
culty of  finding  room  for  signal  guns.  The  gun-cotton 
rocket  completely  surmounts  this  difficulty.  It  is 
manipulated  with  ease  and  rapidity,  while  its  dis- 
charges may  be  so  grouped  and  combined  as  to  give  a 
most  important  extension  to  the  voice  of  the  admiral 
in  command.  It  is  needless  to  add  that  at  any  point 
upon  our  coasts,  or  upon  any  other  coast,  where  its  es- 
tablishment might  be  desirable,  a  fog-signal  station 
might  be  extemporised  without  difficulty. 

I  have  referred  more  than  once  to  the  train  of 
echoes  which  accompanied  the  explosion  of  gun-cotton 
in  free  air,  speaking  of  them  as  similar  in  all  respects 
to  those  which  were  described  for  the  first  time  in  my 
Report  on  Fog-signals,  addressed  to  the  Corporation  of 
Trinity  House  in  1874.*  To  these  echoes  I  attached 
a  fundamental  significance.  There  was  no  visible  re- 
flecting surface  from  which  they  could  come.  On 
some  days,  with  hardly  a  cloud  in  the  air  and  hardly 
a  ripple  on  the  sea,  they  reached  a  magical  intensity. 
As  far  as  the  sense  of  hearing  could  judge,  they  came 
from  the  body  of  the  air  in  front  of  the  great  trumpet 
which  produced  them.  The  trumpet  blasts  were  five 
seconds  in  duration,  but  long  before  the  blast  had 
ceased  the  echoes  struck  in,  adding  their  strength  to 
the  primitive  note  of  the  trumpet.  After  the  blast  had 
ended  the  echoes  continued,  retreating  further  and  fur- 
ther from  the  point  of  observation,  and  finally  dying 
away  at  great  distances.  The  echoes  were  perfectly 
continuous  as  long  as  the  sea  was  clear  of  ships, '  taper- 
ing '  by  imperceptible  gradations  into  absolute  silence. 
But  when  a  ship  happened  to. throw  itself  athwart  the 

*  See  also  '  Philosophical  Transactions'  for  1874,  p.  188. 


276  FRAGMENTS    OF    SCIENCE. 

course  of  the  sound,  the  echo  from  the  broadside  of 
the  vessel  was  returned  as  a  shock  which  rudely  inter- 
rupted the  continuity  of  the  dying  atmospheric  music. 

These  echoes  have  been  ascribed  to  reflection  from 
the  crests  of  the  sea-waves.  But  this  hypothesis  is 
negatived  by  the  fact,  that  the  echoes  were  produced 
in  great  intensity  and  duration  when  no  waves  existed 
— when  the  sea,  in  fact,  was  of  glassy  smoothness.  It 
has  been  also  shown  that  the  direction  of  the  echoes 
depended  not  on  that  of  waves,  real  or  assumed,  but 
on  the  direction  of  the  axis  of  the  trumpet.  Causing 
that  axis  to  traverse  an  arc  of  210°,  and  the  trumpet 
to  sound  at  various  points  of  the  arc,  the  echoes  were 
always,  at  all  events  in  calm  weather,  returned  from 
that  portion  of  the  atmosphere  towards  which  the 
trumpet  was  directed.  They  could  not,  under  the 
circumstances,  come  from  the  glassy  sea;  while  both 
their  variation  of  direction  and  their  perfectly  con- 
tinuous fall  into  silence,  are  irreconcilable  with  the 
notion  that  they  came  from  fixed  objects  on  the  land. 
They  came  from  that  portion  of  the  atmosphere  into 
which  the  trumpet  poured  its  maximum  sound,  and 
fell  in  intensity  as  the  direct  sound  penetrated  to 
greater  atmospheric  distances. 

The  day  on  which  our  latest  observations  were 
made  was  particularly  fine.  Before  reaching  Dunge- 
ness,  the  smoothness  of  the  sea  and  the  serenity  of  the 
air  caused  me  to  test  the  echoing  power  of  the  atmos- 
phere. A  single  ship  lay  about  half  a  mile  distant 
between  us  and  the  land.  The  result  of  the  proposed 
experiment  was  clearly  foreseen.  It  was  this.  The 
rocket  being  sent  up,  it  exploded  at  a  great  height; 
the  echoes  retreated  in  their  usual  fashion,  becoming 
less  and  less  intense  as.  the  distances  of  the  invisible 
surfaces  of  reflection  from  the  observers  increased. 


RECENT    EXPERIMENTS    ON    FOG-SIGNALS.     377 

About  five  seconds  after  the  explosion,  a  single  loud 
shock  was  sent  back  to  us  from  the  side  of  the  vessel 
lying  between  us  and  the  land.  Obliterated  for  a  mo- 
ment by  this  more  intense  echo,  the  aerial  reverbera- 
tion continued  its  retreat,  dying  away  into  silence  in 
two  or  three  seconds  afterwards.* 

I  have  referred  to  the  firing  of  an  8-oz.  rocket  from 
the  deck  of  the  'Galatea'  on  March  8,  1877,  stating 
the  duration  of  its  echoes  to  be  seven  seconds.  Mr. 
Prentice,  who  was  present  at  the  time,  assured  me  that 
in  his  experiments  similar  echoes  had  been  frequently 
heard  of  more  than  twice  this  duration.  The  ranges  of 
his  sounds  alone  would  render  this  result  in  the  high- 
est degree  probable. 

To  attempt  to  interpret  an  experiment  which  I 
have  not  had  an  opportunity  of  repeating,  is  an  opera- 
tion of  some  risk;  and  it  is  not  without  a  consciousness 
of  this  that  I  refer  here  to  a  result  announced  by  Pro- 
fessor Joseph  Henry,  which  he  considers  adverse  to  the 
notion  of  aerial  echoes.  He  took  the  trouble  to  point 
the  trumpet  of  a  syren  towards  the  zenith,  and  found 
that  when  the  syren  was  sounded  no  echo  was  returned. 
Now  the  reflecting  surfaces  which  give  rise  to  these 
echoes  are  for  the  most  part  due  to  differences  of  tem- 
perature between  sea  and  air.  If,  through  any  cause, 
the  air  above  be  chilled,  we  have  descending  streams — 
if  the  air  below  be  warmed,  we  have  ascending  streams 
as  the  initial  cause  of  atmospheric  flocculence.  A 
sound  proceeding  vertically  does  not  cross  the  streams, 
nor  impinge  upon  the  reflecting  surfaces,  as  does  a 
sound  proceeding  horizontally  across  them.  Aerial 
echoes,  therefore,  will  not  accompany  the  vertical  sound 

*  The  echoes  of  the  gun  fired  on  shore  this  day  were  very 
brief ;  those  of  the  12-oz.  gun-cotton  rocket  were  12"  and  those 
of  the  8-oz.  cotton-powder  rocket  11"  in  duration. 


278  FKAGMENTS    OF    SCIENCE. 

as  they  accompany  the  horizontal  one.  The  experi- 
ment, as  I  interpret  it,  is  not  opposed  to  the  theory  of 
these  echoes  which  I  have  ventured  to  enunciate.  But, 
as  I  have  indicated,  not  only  to  see  but  to  vary  such 
an  experiment  is  a  necessary  prelude  to  grasping  its 
full  significance. 

In  a  paper  published  in  the  f  Philosophical  Trans- 
actions '  for  1876,  Professor  Osborne  Eeynolds  refers 
to  these  echoes  in  the  following  terms: — '  Without 
attempting  to  explain  the  reverberations  and  echoes 
which  have  been  observed,  I  will  merely  call  attention 
to  the  fact  that  in  no  case  have  I  heard  any  attending 
the  reports  of  the  rockets,*  although  they  seem  to  have 
been  invariable  with  the  guns  and  pistols.  These  facts 
suggest  that  the  echoes  are  in  some  way  connected  with 
the  direction  given  to  the  sound.  They  are  caused  by 
the  voice,  trumpets,  and  the  syren,  all  of  which  give 
direction  to  the  sound;  but  I  am  not  aware  that  they 
have  ever  been  observed  in  the  case  of  a  sound  which 
has  no  direction  of  greatest  intensity.'  The  reference 
to  the  voice,  and  other  references  in  his  paper,  cause 
me  to  think  that,  in  speaking  of  echoes,  Professor  Os- 
borne Eeynolds  and  myself  are  dealing  with  different 
phenomena.  Be  that  as  it  may,  the  foregoing  observa- 
tions render  it  perfectly  certain  that  the  condition  as 
to  direction  here  laid  down  is  not  necessary  to  the  pro- 
duction of  the  echoes. 

There  is  not  a  feature  connected  with  the  aerial 
echoes  which  cannot  be  brought  out  by  experiments  in 
the  air  of  the  laboratory.  I  have  recently  made  the 
following  experiment: — A  rectangle,  x  Y  (p.  279),  22 
inches  by  12,  was  crossed  by  twenty-three  brass  tubes 
(half  the  number  would  suffice  and  only  eleven  are 

*  These  carried  12  oz.  of  gunpowder,  which  has  been  found  by 
Col.  Fraser  to  require  an  iron  case  to  produce  an  effective  explosion. 


RECENT    EXPERIMENTS    ON    FOG-SIGNALS.     279 

shown  in  the  figure),  each  having  a  slit  along  it  from 
which  gas  can  issue.  In  this  way  twenty-three  low 
flat  flames  were  obtained.  A  sounding  reed  a  fixed  in  a 
short  tube  was  placed  at  one  end  of  the  rectangle,  and 
a  '  sensitive  flame,'  *  /,  at  some  distance  beyond  the 
other  end.  When  the  reed  sounded,  the  flame  in  front 
of  it  was  violently  agitated,  and  roared  boisterously. 
Turning  on  the  gas,  and  lighting  it  as  it  issued  from 
the  slits,  the  air  above  the  flames  became  so  hetero- 


FIG.  0. 


geneous  that  the  sensitive  flame  was  instantly  stilled, 
rising  from  a  height  of  6  inches  to  a  height  of  18 
inches.  Here  we  had  the  acoustic  opacity  of  the  air 
in  front  of  the  South  Foreland  strikingly  imitated,  f 
Turning  off  the  gas,  and  removing  the  sensitive  flame 
to  /',  some  distance  behind  the  reed,  it  burned  there 

*  Fully  described  in  ray  '  Lectures  on  Sound,'  3rd  edition,  p. 
227. 

f  Lectures  on  Sound,  3rd  ed.,  p.  208. 
19 


280  FKAGMENTS    OF    SCIENCE. 

tranquilly,  though  the  reed  was  sounding.  Again 
lighting  the  gas  as  it  issued  from  the  brass  tubes, 
the  sound  reflected  from  the  heterogeneous  air  threw 
the  sensitive  flame  into  violent  agitation.  Here-  we 
had  imitated  the  aerial  echoes  heard  when  standing 
behind  the  syren-trumpet  at  the  South  Foreland.  The 
experiment  is  extremely  simple,  and  in  the  highest 
degree  impressive. 


The  explosive  rapidity  of  dynamite  marks  it  as  a 
substance  specially  suitable  for  the  production  of 
sound.  At  the  suggestion  of  Professor  Dewar,  Mr. 
McRoberts  has  carried  out  a  series  of  experiments  on 
dynamite,  with  extremely  promising  results.  Immedi- 
ately after  the  delivery  of  the  foregoing  lecture  I. was 
informed  that  Mr.  Brock  proposed  the  employment  of 
dynamite  in  the  Collinson  rocket. 


XL 

ON  THE  STUDY  OF  PHYSICS* 

I  HOLD  in  my  hand  an  uncorrected  proof  of  the  syl- 
labus of  this  course  of  lectures,  and  the  title  of 
the  present  lecture  is  there  stated  to  be  '  On  the  Im- 
portance of  the  Study  of  Physics  as  a  Means  of  Educa- 
tion/ The  corrected  proof,  however,  contains  the 
title: — '  On  the  Importance  of  the  Study  of  Physics  as 
a  Branch  of  Education.'  Small  as  this  editorial  altera- 
tion may  seem,  the  two  words  suggest  two  radically  dis- 
tinct modes  of  viewing  the  subject  before  us.  The  term 
Education  is  sometimes  applied  to  a  single  faculty  or 
organ,  and  if  we  know  wherein  the  education  of  a 
single  faculty  consists,  this  will  help  us  to  clearer 
notions  regarding  the  education  of  the  sum  of  all  the 
faculties,  or  of  the  mind.  When,  for  example,  we 
speak  of  the  education  of  the  voice,  what  do  we  mean? 
There  are  certain  membranes  at  the  top  of  the  wind- 
pipe which  throw  into  vibration  the  air  forced  between 
them  from  the  lungs,  thus  producing  musical  sounds. 
These  membranes  are,  to  some  extent,  under  the  con- 
trol of  the  will,  and  it  is  found  that  they  can  be  so 
modified  by  exercise  as  to  produce  notes  of  a  clearer 
and  more  melodious  character.  This  exercise  we  call 
the  education  of  the  voice.  We  may  choose  for  our 
exercise  songs  new  or  old,  festive  or  solemn;  the  edu- 
cation of  the  voice  being  the  object  aimed  at,  the  songs 

*  From  a  lecture  delivered  in  the  Royal  Institution  of  Great 
Britain  in  the  Spring  of  1854. 

281 


282  FRAGMENTS    OF    SCIENCE. 

may  be  regarded  as  the  means  by  which  this  education 
is  accomplished.  I  think  this  expresses  the  state  of 
the  case  more  clearly  than  if  we  were  to  call  the  songs 
a  branch  of  education.  Regarding  also  the  education 
of  the  human  mind  as  the  improvement  and  develop- 
ment of  the  mental  faculties,  I  shall  consider  the  study 
of  Physics  as  a  means  toward  the  attainment  of  this 
end.  From  this  point  of  view,  I  degrade  Physics  into 
an  implement  of  culture,  and  this  is  my  deliberate 
design. 

The  term  Physics,  as  made  use  of  in  the  present 
Lecture,  refers  to  that  portion  of  natural  science  which 
lies  midway  between  astronomy  and  chemistry.  The 
former,  indeed,  is  Physics  applied  to  '  masses  of  enor- 
mous weight,'  while  the  latter  is  Physics  applied  to 
atoms  and  molecules.  The  subjects  of  Physics  proper 
are  therefore  those  which  lie  nearest  to  human  per- 
ception:— light  and  heat,  colour,  sound,  motion,  the 
loadstone,  electrical  attractions  and  repulsions,  thun- 
der and  lightning,  rain,  snow,  dew,  and  so  forth.  Our 
senses  stand  between  these  phenomena  and  the  reason- 
ing mind.  "We  observe  the  fact,  but  are  not  satisfied 
with  the  mere  act  of  observation:  the  fact  must  be  ac- 
counted for — fitted  into  its  position  in  the  line  of  cause 
and  effect.  Taking  our  facts  from  Nature  we  transfer 
them  to  the  domain  of  thought:  look  at  them,  compare 
them,  observe  their  mutual  relations  and  connexions, 
and  bringing  them  ever  clearer  before  the  mental  eye, 
finally  alight  upon  the  cause  which  unites  them.  This 
is  the  last  act  of  the  mind,  in  this  centripetal  direction 
— in  its  progress  from  the  multiplicity  of  facts  to  the 
central  cause  on  which  they  depend.  But,  having 
guessed  the  cause,  we  are  not  yet  contented.  We  set 
out  from  the  centre  and  travel  in  the  other  direction. 
If  the  guess  be  true,  certain  consequences  must  follow 


OX    THE    STUDY    OF    PHYSICS.  283 

from  it,  and  we  appeal  to  the  law  and  testimony  of 
experiment  whether  the  thing  is  so.  Thus  is  the  cir- 
cuit of  thought  completed, — from  without  inward, 
from  multiplicity  to  unity,  and  from  within  outward, 
from  unity  to  multiplicity.  In  thus  traversing  both 
ways  the  line  between  cause  and  effect,  all  our  reason- 
ing powers  are  called  into  play.  The  mental  effort 
involved  in  these  processes  may  be  compared  to  those 
exercises  of  the  body  which  invoke  the  co-operation  of 
every  muscle,  and  thus  confer  upon  the  whole  frame 
the  benefits  of  healthy  action. 

The  first  experiment  a  child  makes  is  a  physical 
experiment:  the  suction-pump  is  but  an  imitation  of 
the  first  act  of  every  new-born  infant.  Nor  do  I  think 
it  calculated  to  lessen  that  infant's  reverence,  or  to 
make  him  a  worse  citizen,  when  his  riper  experience 
shows  him  that  the  atmosphere  was  his  helper  in  ex- 
tracting the  first  draught  from  his  mother's  breast. 
The  child  grows,  but  is  still  an  experimenter:  he  grasps 
at  the  moon,  and  his  failure  teaches  him  to  respect 
distance.  At  length  his  little  fingers  acquire  sufficient 
mechanical  tact  to  lay  hold  of  a  spoon.  He  thrusts 
the  instrument  into  his  mouth,  hurts  his  gums,  and 
thus  learns  the  impenetrability  of  matter.  He  lets  the 
spoon  fall,  and  jumps  with  delight  to  hear  it  rattle 
against  the  table.  The  experiment  made  by  accident 
is  repeated  with  intention,  and  thus  the  young  student 
receives  his  first  lessons  upon  sound  and  gravitation. 
There  are  pains  and  penalties,  however,  in  the  path  of 
the  enquirer:  he  is  sure  to  go  wrong,  and  Nature  is 
just  as  sure  to  inform  him  of  the  fact.  He  falls  down- 
stairs, burns  his  fingers,  cuts  his  hand,  scalds  his 
tongue,  and  in  this  way  learns  the  conditions  of  his 
physical  well  being.  This  is  Nature's  way  of  proceed- 
ing, and  it  is  wonderful  what  progress  her  pupil  makes. 


284  FRAGMENTS    OF    SCIENCE. 

His  enjoyments  for  a  time  are  physical,  and  the  con- 
fectioner's shop  occupies  the  foreground  of  human 
happiness;  but  the  blossoms  of  a  finer  life  are  already 
beginning  to  unfold  themselves,  and  the  relation  of 
cause  and  effect  dawns  upon  the  boy.  He  begins  to 
see  that  the  present  condition  of  things  is  not  final, 
but  depends  upon  one  that  has  gone  before,  and  will 
be  succeeded  by  another.  He  becomes  a  puzzle  to  him- 
self; and  to  satisfy  his  newly-awakened  curiosity,  asks 
all  manner  of  inconvenient  questions.  The  needs  and 
tendencies  of  human  nature  express  themselves  through 
these  early  yearnings  of  the  child.  As  thought  ripens, 
he  desires  to  know  the  character  and  causes  of  the 
phenomena  presented  to  his  observation;  and  unless 
this  desire  has  been  granted  for  the  express  purpose  of 
having  it  repressed,  unless  the  attractions  of  natural 
phenomena  be  like  the  blush  of  the  forbidden  fruit, 
conferred  merely  for  the  purpose  of  exercising  our  self- 
denial  in  letting  them  alone;  we  may  fairly  claim  for 
the  study  of  Physics  the  recognition  that  it  answers  to 
an  impulse  implanted  by  nature  in  the  constitution 
of  man. 

A  few  days  ago,  a  Master  of  Arts,  who  is  still  a 
young  man,  and  therefore  the  recipient  of  a  modern 
education,  stated  to  me  that  until  he  had  reached  the 
age  of  twenty  years  he  had  never  been  taught  anything 
whatever  regarding  natural  phenomena,  or  natural  law. 
Twelve  years  of  his  life  previously  had  been  spent 
exclusively  among  the  ancients.  The  case,  I  regret  to 
say,  is  typical.  Now,  we  cannot,  without  prejudice  to 
humanity,  separate  the  present  from  the  past.  The 
nineteenth  century  strikes  its  roots  into  the  centuries 
gone  by,  and  draws  nutriment  from  them.  The  world 
cannot  afford  to  lose  the  record  of  any  great  deed  or 
utterance;  for  such  are  prolific  throughout  all  time. 


ON    THE   STUDY   OF   PHYSICS.  285 

We  cannot  yield  the  companionship  of  our  loftier 
brothers  of  antiquity, — of  our  Socrates  and  Cato, — 
whose  lives  provoke  us  to  sympathetic  greatness  across 
the  interval  of  two  thousand  years.  As  long  as  the 
ancient  languages  are  the  means  of  access  to  the  an- 
cient mind,  they  must  ever  be  of  priceless  value  to  hu- 
manity; but  surely  these  avenues  might  be  kept  open 
without  making  such  sacrifices  as  that  above  referred 
to,  universal.  We  have  conquered  and  possessed  our- 
selves of  continents  of  land,  concerning  which  antiqui- 
ty knew  nothing;  and  if  new  continents  of  thought 
reveal  themselves  to  the  exploring  human  spirit,  shall 
we  not  possess  them  also?  In  these  latter  days,  the 
study  of  Physics  has  given  us  glimpses  of  the  methods 
of  Nature  which  were  quite  hidden  from  the  ancients, 
and  we  should  be  false  to  the  trust  committed  to  us, 
if  we  were  to  sacrifice  the  hopes  and  aspirations  of  the 
Present  out  of  deference  to  the  Past. 

The  bias  of  my  own  education  probably  manifests 
itself  in  a  desire  I  always  feel  to  seize  upon  every  pos- 
sible opportunity  of  checking  my  assumptions  and 
conclusions  by  experience.  In  the  present  case,  it  is 
true,  your  own  consciousness  might  be  appealed  to  in 
proof  of  the  tendency  of  the  human  mind  to  inquire 
into  the  phenomena  presented  to  it  by  the  senses;  but 
I  trust  you  will  excuse  me  if,  instead  of  doing  this,  I 
take  advantage  of  the  facts  which  have  fallen  in  my 
way  through  life,  referring  to  your  judgment  to  de- 
cide whether  such  facts  are  truly  representative  and 
general,  and  not  merely  individual  and  local. 

At  an  agricultural  college  in  Hampshire,  with 
which  I  was  connected  for  some  time,  and  which  is 
now  converted  into  a  school  for  the  general  education 
of  youth,  a  Society  was  formed  among  the  boys,  who 
met  weekly  for  the  purpose  of  reading  reports  and 


286  FKAGMENTS    OF    SCIENCE. 

papers  upon  various  subjects.  The  Society  had  its 
president  and  treasurer;  and  abstracts  of  its  proceed- 
ings were  published  in  a  little  monthly  periodical  issu- 
ing from  the  school  press.  One  of  the  most  remark- 
able features  of  these  weekly  meetings  was,  that  after 
the  general  business  had  been  concluded,  each  member 
enjoyed  the  right  of  asking  questions  on  any  subject 
on  which  he  desired  information.  The  questions  were 
either  written  out  previously  in  a  book,  or,  if  a  ques- 
tion happened  to  suggest  itself  during  the  meeting,  it 
was  written  upon  a  slip  of  paper  and  handed  in  to  the 
Secretary,  who  afterwards  read  all  the  questions  aloud. 
A  number  of  teachers  were  usually  present,  and  they 
and  the  boys  made  a  common  stock  of  their  wisdom 
in  furnishing  replies.  As  might  be  expected  from  an 
assemblage  of  eighty  or  ninety  boys,  varying  from 
eighteen  to  eight  years  old,  many  odd  questions  were 
proposed.  To  the  mind  which  loves  to  detect  in  the 
tendencies  of  the  young  the  instincts  of  humanity 
generally,  such  questions  are  not  without  a  certain 
philosophic  interest,  and  I  have  therefore  thought  it 
not  derogatory  to  the  present  course  of  Lectures  to 
copy  a  few  of  them,  and  to  introduce  them  here.  They 
run  as  follows: —  / 

What  are  the  duties  of  the  Astronomer  Eoyal? 

What  is  frost? 

Why  are  thunder  and  lightning  more  frequent  in 
summer  than  in  winter? 

What  occasions  falling  stars? 

What  is  the  cause  of  the  sensation  called  *  pins  and 
needles'? 

What  is  the  cause  of  waterspouts? 

What  is  the  cause  of  hiccup? 

If  a  towel  be  wetted  with  water,  why  does  the  wet 
portion  become  darker  than  before? 


ON   THE   STUDY   OF   PHYSICS.  287 

What  is  meant  by  Lancashire  witches? 

Does  the  dew  rise  or  fall? 

What  is  the  principle  of  the  hydraulic  press? 

Is  there  more  oxygen  in  the  air  in  summer  than  in 
winter? 

What  are  those  rings  which  we  see  round  the  gas 
and  sun? 

What  is  thunder? 

How  is  it  that  a  black  hat  can  be  moved  by  forming 
round  it  a  magnetic  circle,  while  a  white  hat  remains 
stationary? 

What  is  the  cause  of  perspiration? 

Is  it  true  that  men  were  once  monkeys? 

What  is  the  difference  between  the  soul  and  the 
mind? 

Is  it  contrary  to  the  rules  of  Vegetarianism  to 
eat  eggs? 

In  looking  over  these  questions,  which  were  wholly 
unprompted,  and  have  been  copied  almost  at  random 
from  the  book  alluded  to,  we  see  that  many  of  them 
are  suggested  directly  by  natural  objects,  and  are  not 
such  as  had  an  interest  conferred  on  them  by  previous 
culture.  Now  the  fact  is  beyond  the  boy's  control,  and 
so  certainly  is  the  desire  to  know  its  cause.  The  sole 
question  then  is,  whether  this  desire  is  to  be  gratified 
or  not.  Who  created  the  fact?  Who  implanted  the 
desire?  Certainly  not  man.  Who  then  will  undertake 
to  place  himself  between  the  desire  and  its  fulfilment, 
and  proclaim  a  divorce  between  them?  Take,  for  ex- 
ample, the  case  of  the  wetted  towel,  which  at  first  sight 
appears  to  be  one  of  the  most  unpromising  questions 
in  the  list.  Shall  we  tell  the  proposer  to  repress  his 
curiosity,  as  the  subject  is  improper  for  him  to  know, 
and  thus  interpose  our  wisdom  to  rescue  the  boy  from 
the  consequences  of  a  wish  which  acts  to  his  prejudice? 


288  FRAGMENTS   OF   SCIENCE. 

Or,  recognising  the  propriety  of  the  question,  how 
shall  we  answer  it?  It  is  impossible  to  answer  it  with- 
out reference  to  the  laws  of  optics — without  making 
the  boy  to  some  extent  a  natural  philosopher.  You 
may  say  that  the  effect  is  due  to  the  reflection  of  light 
at  the  common  surface  of  two  media  of  different  re- 
fractive indices.  But  this  answer  presupposes  on  the 
part  of  the  boy  a  knowledge  of  what  reflection  and 
refraction  are,  or  reduces  you  to  the  necessity  of  ex- 
plaining them. 

On  looking  more  closely  into  the  matter,  we  find 
that  our  wet  towel  belongs  to  a  class  of  phenomena 
which  have  long  excited  the  interest  of  philosophers. 
The  towel  is  white  for  the  same  reason  that  snow  is 
white,  that  foam  is  white,  that  pounded  granite  or 
glass  is  white,  and  that  the  salt  we  use  at  table  .is 
white.  On  quitting  one  medium  and  entering  another, 
a  portion  of  light  is  always  reflected,  but  on  this  condi- 
tion— the  media  must  possess  different  refractive  in- 
dices. Thus,  when  we  immerse  a  bit  of  glass  in  water, 
light  is  reflected  from  the  common  surface  of  both,  and 
it  is  this  light  which  enables  us  to  see  the  glass.  But 
when  a  transparent  solid  is  immersed  in  a  liquid  of  the 
same  refractive  index  as  itself,  it  immediately  disap- 
pears. I  remember  once  dropping  the  eyeball  of  an 
ox  into  water;  it  vanished  as  if  by  magic,  with  the 
exception  of  the  crystalline  lens,  and  the  surprise  was 
so  great  as  to  cause  a  bystander  to  suppose  that  the 
vitreous  humour  had  been  instantly  dissolved.  This, 
however,  was  not  the  case,  and  a  comparison  of  the 
refractive  index  of  the  humour  with  that  of  water 
cleared  up  the  whole  matter.  The  indices  were  identi- 
cal, and  hence  the  light  pursued  its  way  through  both 
as  if  they  formed  one  continuous  mass. 

In  the  case  of  snow,  powdered  quartz,  or  salt,  we 


ON   THE    STUDY    OF   PHYSICS.  289 

have  a  transparent  solid  mixed  with  air.  At  every 
transition  from  solid  to  air,  or  from  air  to  solid,  a  por- 
tion of  light  is  reflected,  and  this  takes  place  so  often 
that  the  light  is  wholly  intercepted.  Thus  from  the 
mixture  of  two  transparent  bodies  we  obtain  an  opaque 
one.  Now  the  case  of  the  towel  is  precisely  similar. 
The  tissue  is  composed  of  semi-transparent  vegetable 
fibres,  with  the  interstices  between  them  filled  with 
air;  repeated  reflection  takes  place  at  the  limiting 
surfaces  of  air  and  fibre,  and  hence  the  towel  becomes 
opaque  like  snow  or  salt.  But  if  we  fill  the  interstices 
with  water,  we  diminish  the  reflection;  a  portion  of 
the  light  is  transmitted,  and  the  darkness  of  the  towel 
is  due  to  its  increased  transparency.  Thus  the  deport- 
ment of  various  minerals,  such  as  hydrophane  and 
tabasheer,  the  transparency  of  tracing  paper  used  by 
engineers,  and  many  other  considerations  of  the  high- 
est scientific  interest,  are  involved  in  the  simple  en- 
quiry of  this  unsuspecting  little  boy. 

Again,  take  the  question  regarding  the  rising  or 
falling  of  the  dew — a  question  long  agitated,  and  final- 
ly set  at  rest  by  the  beautiful  researches  of  Wells.  I  do 
not  think  that  any  boy  of  average  intelligence  will  be 
satisfied  with  the  simple  answer  that  the  dew  falls. 
He  will  wish  to  learn  how  you  know  that  it  falls,  and, 
if  acquainted  with  the  notions  of  the  middle  ages,  he 
may  refer  to  the  opinion  of  Father  Laurus,  that  a  gooso 
egg  filled  in  the  morning  with  dew  and  exposed  to  the 
sun,  will  rise  like  a  balloon — a  swan's  egg  being  better 
for  the  experiment  than  a  goose  egg.  It  is  impossible 
to  give  the  boy  a  clear  notion  of  the  beautiful  phe- 
nomenon to  which  his  question  refers,  without  first 
making  him  acquainted  with  the  radiation  and  con- 
duction of  heat.  Take,  for  example,  a  blade  of  grass, 
from  which  one  of  these  orient  pearls  is  depending, 


290  FBAGMENTS    OF    SCIENCE. 

During  the  day  the  grass,  and  the  earth  beneath  it, 
possess  a  certain  amount  of  warmth  imparted  by  the 
sun;  during  a  serene  night,  heat  is  radiated  from  the 
surface  of  the  grass  into  space,  and  to  supply  the  loss, 
there  is  a  flow  of  heat  from  the  earth  to  the  blade. 
Thus  the  blade  loses  heat  by  radiation,  and  gains  heat 
by  conduction.  Now,  in  the  case  before  us,  the  power 
of  radiation  is  great,  whereas  the  power  of  conduction 
is  small;  the  consequence  is  that  the  blade  loses  more 
than  it  gains,  and  hence  becomes  more  and  more  re- 
frigerated. The  light  vapour  floating  around  the  sur- 
face so  cooled  is  condensed  upon  it,  and  there  accumu- 
lates to  form  the  little  pearly  globe  which  we  call  a 
dew-drop. 

Thus  the  boy  finds  the  simple  and  homely  fact 
which  addressed  his  senses  to  be  the  outcome  and  flower 
of  the  deepest  laws.  The  fact  becomes,  in  a  measure, 
sanctified  as  an  object  of  thought,  and  invested  for  him 
with  a  beauty  for  evermore.  He  thus  learns  that 
things  which,  at  first  sight,  seem  to  stand  isolated  and 
without  apparent  brotherhood  in  Nature  are  organic- 
ally united,  and  finds  the  detection  of  such  analogies 
a  source  of  perpetual  delight.  To  enlist  pleasure  on 
the  side  of  intellectual  performance  is  a  point  of  the 
utmost  importance;  for  the  exercise  of  the  mind,  like 
that  of  the  body,  depends  for  its  value  upon  the  spirit 
in  which  it  is  accomplished.  Every  physician  knows 
that  something  more  than  mere  mechanical  motion  is 
comprehended  under  the  idea  of  healthful  exercise — 
that,  indeed,  being  most  healthful  which  makes  us 
forget  all  ulterior  ends  in  the  mere  enjoyment  of  it. 
What,  for  example,  could  be  substituted  for  the  action 
of  the  playground,  where  the  boy  plays  for  the  mere 
love  of  playing,  and  without  reference  to  physiological 
laws;  while  kindly  Nature  accomplishes  her  ends  un- 


ON    THE    STUDY    OF    PHYSICS.  291 

consciously,  and  makes  his  very  indifference  beneficial 
to  him.  You  may  have  more  systematic  motions,  you 
may  devise  means  for  the  more  perfect  traction  of  each 
particular  muscle,  but  you  cannot  create  the  joy  and 
gladness  of  the  game,  and  where  these  are  absent,  the 
charm  and  the  health  of  the  exercise  are  gone.  The 
case  is  similar  with  the  education  of  the  mind. 

The  study  of  Physics,  as  already  intimated,  consists 
of  two  processes,  which  are  complementary  to  each 
other — the  tracing  of  facts  to  their  causes,  and  the 
logical  advance  from  the  cause  to  the  fact.  In  the 
former  process,  called  induction,  certain  moral  qualities 
come  into  play.  The  first  condition  of  success  is  pa- 
tient industry,  an  honest  receptivity,  and  a  willingness 
to  abandon  all  preconceived  notions,  however  cher- 
ished, if  they  be  found  to  contradict  the  truth.  Be- 
lieve me,  a  self-renunciation  which  has  something  lofty 
in  it,  and  of  which  the  world  never  hears,  is  often  en- 
acted in  the  private  experience  of  the  true  votary  of 
science.  And  if  a  man  be  not  capable  of  this  self- 
renunciation — this  loyal  surrender  of  himself  to  Na- 
ture and  to  fact,  he  lacks,  in  my  opinion,  the  first  mark 
of  a  true  philosopher.  Thus  the  earnest  prosecutor  of 
science,  who  does  not  work  with  the  idea  of  producing 
a  sensation  in  the  world,  who  loves  the  truth  better 
than  the  transitory  blaze  of  to-day's  fame,  who  comes 
to  his  task  with  a  single  eye,  finds  in  that  task  an  in- 
direct means  of  the  highest  moral  culture.  And  al- 
though the  virtue  of  the  act  depends  upon  its  privacy, 
this  sacrifice  of  self,  this  upright  determination  to  ac- 
cept the  truth,  no  matter  how  it  may  present  itself — 
even  at  the  hands  of  a  scientific  foe,  if  necessary — car- 
ries with  it  its  own  reward.  When  prejudice  is  put 
under  foot  and  the  stains  of  personal  bias  have  been 
washed  away — when  a  man  consents  to  lay  aside  his 


292  FRAGMENTS    OF    SCIENCE. 

vanity  and  to  become  Nature's  organ — his  elevation  is 
the  instant  consequence  of  his  humility.  I  should  not 
wonder  if  my  remarks  provoked  a  smile,  for  they  seem 
to  indicate  that  I  regard  the  man  of  science  as  a  heroic, 
if  not  indeed  an  angelic,  character;  and  cases  may 
occur  to  you  which  indicate  the  reverse.  You  may 
point  to  the  quarrels  of  scientific  men,  to  their  strug- 
gles for  priority,  to  that  unpleasant  egotism  which 
screams  around  its  little  property  of  discovery  like  a 
scared  plover  about  its  young.  I  will  not  deny  all  this; 
but  let  it  be  set  down  to  its  proper  account,  to  the 
weakness — or,  if  you  will — to  the  selfishness  of  Man, 
but  not  to  the  charge  of  Physical  Science. 

The  second  process  in  physical  investigation  is  de- 
duction, or  the  advance  of  the  mind  from  fixed  prin- 
ciples to  the  conclusions  which  flow  from  them.  The 
rules  of  logic  are  the  formal  statement  of  this  process, 
which,  however,  was  practised  by  every  healthy  mind 
before  ever  such  rules  were  written.  In  the  study  of 
Physics,  induction  and  deduction  are  perpetually 
wedded  to  each  other.  The  man  observes,  strips  facts 
of  their  peculiarities  of  form,  and  tries  to  unite  them 
by  their  essences;  having  effected  this,  he  at  once  de- 
duces, and  thus  checks  his  induction.  Here  the  grand 
difference  between  the  methods  at  present  followed, 
and  those  of  the  ancients,  becomes  manifest.  They 
were  one-sided  in  these  matters:  they  omitted  the 
process  of  induction,  and  substituted  conjecture  for 
observation.  They  could  never,  therefore,,  fulfil  the 
mission  of  Man  to  '  replenish  the  earth,  and  subdue  it.' 
The  subjugation  of  Nature  is  only  to  be  accomplished 
by  the  penetration  of  her  secrets  and  the  patient  mas- 
tery of  her  laws.  This  not  only  enables  us  to  protect 
ourselves  from  the  hostile  action  of  natural  forces,  but 
makes  them  our  slaves.  By  the  study  of  Physics  we 


ON    THE    STUDY    OF    PHYSICS.  293 

have  indeed  opened  to  us  treasuries  of  power  of  which 
antiquity  never  dreamed.  But  while  we  lord  it  over 
Matter,  we  have  thereby  become  better  acquainted  with 
the  laws  of  Mind;  for  to  the  mental  philosopher  the 
study  of  Physics  furnishes  a  screen  against  which  the 
human  spirit  projects  its  own  image,  and  thus  be- 
comes capable  of  self-inspection. 

Thus,  then,  as  a  means  of  intellectual  culture,  the 
study  of  Physics  exercises  and  sharpens  observation:  it 
brings  the  most  exhaustive  logic  into  play:  it  com- 
pares, abstracts,  and  generalizes,  and  provides  a  mental 
scenery  appropriate  to  these  processes.  The  strictest 
precision  of  thought  is  everywhere  enforced,  and  pru- 
dence, foresight,  and  sagacity  are  demanded.  By  its 
appeals  to  experiment,  it  continually  checks  itself,  and 
thus  walks  on  a  foundation  of  facts.  Hence  the  exer- 
cise it  invokes  does  not  end  in  a  mere  game  of  intel- 
lectual gymnastics,  such  as  the  ancients  delighted  in, 
but  tends  to  the  mastery  of  Nature.  This  gradual  con- 
quest of  the  external  world,  and  the  consciousness  of 
augmented  strength  which  accompanies  it,  render  the 
study  of  Physics  as  delightful  as  it  is  important. 

With  regard  to  the  effect  on  the  imagination,  cer- 
tain it  is  that  the  cool  results  of  physical  induction 
furnish  conceptions  which  transcend  the  most  daring 
flights  of  that  faculty.  Take  for  example  the  idea  of 
an  all-pervading  ether  which  transmits  a  tingle,  so  to 
speak,  to  the  finger  ends  of  the  universe  every  time  a 
street  lamp  is  lighted.  The  invisible  billows  of  this 
ether  can  be  measured  with  the  same  ease  and  certainty 
as  that  with  which  an  engineer  measures  a  base  and 
two  angles,  and  from  these  finds  the  distance  across  the 
Thames.  Now  it  is  to  be  confessed  that  there  may  be 
just  as  little  poetry  in  the  measurement  of  an  ethereal 
undulation  as  in  that  of  the  river;  for  the  intellect, 


294  FRAGMENTS    OF    SCIENCE. 

during  the  acts  of  measurement  and  calculation,  de- 
stroys those  notions  of  size  which  appeal  to  the  poetic 
sense.  It  is  a  mistake  to  suppose,  with  Dr.  Young,  that 

An  undevout  astronomer  is  mad ; 

there  being  no  necessary  connexion  between  a  devout 
state  of  mind  and  the  observations  and  calculations  of 
a  practical  astronomer.  It  is  not  until  the  man  with- 
draws from  his  calculation,  as  a  painter  from  his  work, 
and  thus  realizes  the  great  idea  on  which  he  has  been 
engaged,  that  imagination  and  wonder  are  excited. 
There  is,  I  admit,  a  possible  danger  here.  If  the  arith- 
metical processes  of  science  be  too  exclusively  pursued, 
they  may  impair  the  imagination,  and  thus  the  study 
of  Physics  is  open  to  the  same  objection  as  philological, 
theological,  or  political  studies,  when  carried  to  excess. 
But  even  in  this  case,  the  injury  done  is  to  the  in- 
vestigator himself:  it  does  not  reach  the  mass  of  man- 
kind. Indeed,  the  conceptions  furnished  by  his  cold 
unimaginative  reckonings  may  furnish  themes  for  the 
poet,  and  excite  in  the  highest  degree  that  sentiment 
of  wonder  which,  notwithstanding  all  its  foolish  vaga- 
ries, table-turning  included,  I,  for  my  part,  should  be 
sorry  to  see  banished  from  the  world. 

I  have  thus  far  dwelt  upon  the  study  of  Physics  as 
an  agent  of  intellectual  culture;  but  like  other  things 
in  Nature,  this  study  subserves  more  than  a  single  end. 
The  colours  of  the  clouds  delight  the  eye,  and,  no 
doubt,  accomplish  moral  purposes  also,  but  the  self- 
same clouds  hold  within  their  fleeces  the  moisture  by 
which  our  fields  are  rendered  fruitful.  The  sunbeams 
excite  our  interest  and  invite  our  investigation;  but 
they  also  extend  their  beneficent  influences  to  our 
fruits  and  corn,  and  thus  accomplish,  not  only  intel-. 
lectual  ends,  but  minister,  at  the  same  time,  to  our 


ON   THE   STUDY   OF   PHYSICS.  295 

material  necessities.  And  so  it  is  with  scientific  re- 
search. While  the  love  of  science  is  a  sufficient  incen- 
tive to  the  pursuit  of  science,  and  the  investigator,  in 
the  prosecution  of  his  enquiries,  is  raised  above  all 
material  considerations,  the  results  of  his  labours  may 
exercise  a  potent  influence  upon  the  physical  condition 
of  the  community.  This  is  the  arrangement  of  Nature, 
and  not  that  of  the  scientific  investigator  himself;  for 
he  usually  pursues  his  object  without  regard  to  its 
practical  applications. 

And  let  him  who  is  dazzled  by  such  applications — 
who  sees  in  the  steam-engine  and  the  electric  telegraph 
the  highest  embodiment  of  human  genius  and  the  only 
legitimate  object  of  scientific  research,  beware  of  pre- 
scribing conditions  to  the  investigator.  Let  him  be- 
ware of  attempting  to  substitute  for  that  simple  love 
with  which  the  votary  of  science  pursues  his  task,  the 
calculations  of  what  he  is  pleased  to  call  utility.  The 
professed  utilitarian  is  unfortunately,  in  most  cases, 
the  very  last  man  to  see  the  occult  sources  from  which 
useful  results  are  derived.  He  admires  the  flower,  but 
is  ignorant  of  the  conditions  of  its  growth.  The  scien- 
tific man  must  approach  Nature  in  his  own  way;  for 
if  you  invade  his  freedom  by  your  so-called  practical 
considerations,  it  may  be  at  the  expense  of  those  quali- 
ties on  which  his  success  as  a  discoverer  depends.  Let 
the  self-styled  practical  man  look  to  those  from  the 
fecundity  of  whose  thought  he,  and  thousands  like 
him,  have  sprung  into  existence.  Were  they  inspired 
in  their  first  enquiries  by  the  calculations  of  utility? 
Not  one  of  them.  They  were  often  forced  to  live  low 
and  lie  hard,  and  to  seek  compensation  for  their  pen- 
ury in  the  delight  which  their  favourite  pursuits  af- 
forded them.  In  the  words  of  one  well  qualified  to 
speak  upon  this  subject,  *  I  say  not  merely  look  at  the 


296  FRAGMENTS    OF    SCIENCE. 

pittance  of  men  like  John  Dalton,  or  the  voluntary 
starvation  of  the  late  Graff;  but  compare  what  is  con- 
sidered as  competency  or  affluence  by  your  Faradays, 
Liebigs,  and  Ilerschels,  with  the  expected  results  of  a 
life  of  successful  commercial  enterprise:  then  compare 
the  amount  of  mind  put  forth,  the  work  done  for  soci- 
ety in  either  case,  and  you  will  be  constrained  to  allow 
that  the  former  belong  to  a  class  of  workers  who,  prop- 
erly speaking,  are  not  paid,  and  cannot  be  paid  for 
their  work,  as  indeed  it  is  of  a  sort  to  which  no  pay- 
ment could  stimulate.' 

But  while  the  scientific  investigator,  standing  upon 
the  frontiers  of  human  knowledge,  and  aiming  at  the 
conquest  of  fresh  soil  from  the  surrounding  region 
of  the  unknown,  makes  the  discovery  of  truth  his  ex- 
clusive object  for  the  time,  he  cannot  but  feel  the 
deepest  interest  in  the  practical  application  of  the 
truth  discovered.  There  is  something  ennobling  in 
the  triumph  of  Mind  over  Matter.  Apart  even  from  its 
uses  to  society,  there  is  something  elevating  in  the  idea 
of  Man  having  tamed  that  wild  force  which  flashes 
through  the  telegraphic  wire,  and  made  it  the  minis- 
ter of  his  will.  Our  attainments  in  these  directions 
appear  to  be  commensurate  with  our  needs.  We  had 
already  subdued  horse  and  mule,  and  obtained  from 
them  all  the  service  which  it  was  in  their  power  to  ren- 
der: we  must  either  stand  still,  or  find  more  potent 
agents  to  execute  our  purposes.  At  this  point  the 
steam-engine  appears.  These  are  still  new  things;  it 
is  not  long  since  we  struck  into  the  scientific  methods 
which  have  produced  these  results.  We  cannot  for  an 
instant  regard  them  as  the  final  achievements  of  Sci- 
ence, but  rather  as  an  earnest  of  what  she  is  yet  to  do. 
They  mark  our  first  great  advances  upon  the  dominion 
of  Nature.  Animal  strength  fails,  but  here  are  the 


ON   THE    STUDY   OF   PHYSICS.  397 

forces  which  hold  the  world  together,  and  the  instincts 
and  successes  of  Man  assure  him  that  these  forces  are 
his  when  he  is  wise  enough  to  command  them. 

As  an  instrument  of  intellectual  culture,  the  study 
of  Physics  is  profitable  to  all:  as  bearing  upon  special 
functions,  its  value,  though  not  so  great,  is  still  more 
tangible.  Why,  for  example,  should  Members  of  Par- 
liament be  ignorant  of  the  subjects  concerning  which 
they  are  called  upon  to  legislate?  In  this  land  of 
practical  physics,  why  should  they  be  unable  to  form 
an  independent  opinion  upon  a  physical  question? 
Why  should  the  member  of  a  parliamentary  committee 
be  left  at  the  mercy  of  interested  disputants  when  a 
scientific  question  is  discussed,  until  he  deems  the  nap 
a  blessing  which  rescues  him  from  the  bewilderments 
of  the  committee-room?  The  education  which  does 
not  supply  the  want  here  referred  to,  fails  in  its  duty 
to  England.  With  regard  to  our  working  people,  in 
the  ordinary  sense  of  the  term  working,  the  study  of 
Physics  would,  I  imagine,  be  profitable,  not  only  as  a 
means  of  intellectual  culture,  but  also  as  a  moral  in- 
fluence to  woo  them  from  pursuits  which  now  degrade 
them.  A  man's  reformation  oftener  depends  upon  the 
indirect,  than  upon  the  direct  action  of  the  will.  The 
will  must  be  exerted  in  the  choice  of  employment 
which  shall  break  the  force  of  temptation  by  erecting 
a  barrier  against  it.  The  drunkard,  for  example,  is  in 
a  perilous  condition  if  he  content  himself  merely  with 
saying,  or  swearing,  that  he  will  avoid  strong  drink. 
His  thoughts,  if  not  attracted  by  another  force,  will 
revert  to  the  public-house,  and  to  rescue  him  perma- 
nently from  this,  you  must  give  him  an  equivalent. 

By  investing  the  objects  of  hourly  intercourse  with 
an  interest  which  prompts  reflection,  new  enjoyments 
would  be  opened  to  the  working  man,  and  every  one 


298  FRAGMENTS    OF    SCIENCE. 

of  these  would  be  a  point  of  force  to  protect  him 
against  temptation.  Besides  this,  our  factories  and  our 
foundries  present  an  extensive  field  of  observation,  and 
were  those  who  work  in  them  rendered  capable,  by 
previous  culture,  of  observing  what  they  see,  the  results 
might  be  incalculable.  Who  can  say  what  intellectual 
Samsons  are  at  the  present  moment  toiling  with  closed 
eyes  in  the  mills  and  forges  of  Manchester  and  Bir- 
mingham? Grant  these  Samsons  sight,  and  you  multi- 
ply the  chances  of  discovery,  and  with  them  the  pros- 
pects of  national  advancement.  In  our  multitudinous 
technical  operations  we  are  constantly  playing  with 
forces  our  ignorance  of  which  is  often  the  cause  of 
our  destruction.  There  are  agencies  at  work  in  a 
loeomotive  of  which  the  maker  of  it  probably  never 
dreamed,  but  which  nevertheless  may  be  sufficient 
to  convert  it  into  an  engine  of  death.  When  we  reflect 
on  the  intellectual  condition  of  the  people  who  work 
in  our  coal  mines,  those  terrific  explosions  which  occur 
from  time  to  time  need  not  astonish  us.  If  these  men 
possessed  sufficient  physical  knowledge,  from  the  op- 
eratives themselves  would  probably  emanate  a  system 
by  which  these  shocking  accidents  might  be  avoided. 
Possessed  of  the  knowledge,  their  personal  interests 
would  furnish  the  necessary  stimulus  to  its  practical 
application,  and  thus  two  ends  would  be  served  at  the 
same  time — the  elevation  of  the  men  and  the  diminu- 
tion of  the  calamity. 

Before  the  present  Course  of  Lectures  was  publicly 
announced,  I  had  many  misgivings  as  to  the  propriety 
of  my  taking  a  part  in  them,  thinking  that  my  place 
might  be  better  filled  by  an  older  and  more  experienced 
man.  To  my  experience,  however,  such  as  it  was,  I 
resolved  to  adhere,  and  I  have  therefore  described 
things  as  they  revealed  themselves  to  my  own  eyes,  and 


ON   THE   STUDY   OF   PHYSICS.  299 

have  been  enacted  in  my  own  limited  practice.  There 
is  one  mind  common  to  us  all;  and  the  true  expression 
of  this  mind,  even  in  small  particulars,  will  attest  itself 
by  the  response  which  it  calls  forth  in  the  convictions 
of  my  hearers.  I  ask  your  permission  to  proceed  a 
little  further  in  this  fashion,  and  to  refer  to  a  fact  or 
two  in  addition  to  those  already  cited,  which  presented 
themselves  to  my  notice  during  my  brief  career  as  a 
teacher  in  the  college  already  alluded  to.  The  facts, 
though  extremely  humble,  and  deviating  in  some 
slight  degree  from  the  strict  subject  of  the  present  dis- 
course, may  yet  serve  to  illustrate  an  educational  prin- 
ciple. 

One  of  the  duties  which  fell  to  my  share  was  the 
instruction  of  a  class  in  mathematics,  and  I  usually 
found  that  Euclid  and  the  ancient  geometry  generally, 
when  properly  and  sympathetically  addressed  to  the 
understanding,  formed  a  most  attractive  study  for 
youth.  But  it  was  my  habitual  practice  to  withdraw 
the  boys  from  the  routine  of  the  book,  and  to  appeal 
to  their  self-power  in  the  treatment  of  questions  not 
comprehended  in  that  routine.  At  first,  the  change 
from  the  beaten  track  usually  excited  aversion:  the 
youth  felt  like  a  child  amid  strangers;  but  in  no  sin- 
gle instance  did  this  feeling  continue.  When  utterly 
disheartened,  I  have  encouraged  the  boy  oy  the  anec- 
dote of  Newton,  where  he  attributes  the  difference 
between  him  and  other  men,  mainly  to  his  own  pa- 
tience; or  of  Mirabeau,  when  he  ordered  his  servant, 
who  had  stated  something  to  be  impossible,  never  again 
to  use  that  blockhead  of  a  word.  Thus  cheered,  the 
boy  has  returned  to  his  task  with  a  smile,  which  per- 
haps had  something  of  doubt  in  it,  but  which,  never- 
theless, evinced  a  resolution  to  try  again.  I  have  seen 
his  eye  brighten,  and,  at  length,  with  a  pleasure  of 


300  FEAGMENTS    OF    SCIENCE. 

which  the  ecstasy  of  Archimedes  was  but  a  simple  ex- 
pansion, heard  him  exclaim,  '  I  have  it,  sir.'  The  con- 
sciousness of  self-power,  thus  awakened,  was  of  im- 
mense value;  and,  animated  by  it,  the  progress  of  the 
class  was  astonishing.  It  was  often  my  custom  to  give 
the  boys  the  choice  of  pursuing  their  propositions  in 
the  book,  or  of  trying  their  strength  at  others  not  to 
be  found  there.  Never  in  a  single  instance  was  the 
book  chosen.  I  was  ever  ready  to  assist  when  help 
was  needful,  but  my  offers  of  assistance  were  habit- 
ually declined.  The  boys  had  tasted  the  sweets  of  in- 
tellectual conquest  and  demanded  victories  of  their 
own.  Their  diagrams  were  scratched  on  the  walls,  cut 
into  the  beams  upon  the  playground,  and  numberless 
other  illustrations  were  afforded  of  the  living  interest 
they  took  in  the  subject.  For  my  own  part,  as  far  as 
experience  in  teaching  goes,  I  was  a  mere  fledgling — 
knowing  nothing  of  the  rules  of  pedagogics,  as  the 
Germans  name  it;  but  adhering  to  the  spirit  indi- 
cated at  the  commencement  of  this  discourse,  and  en- 
deavouring to  make  geometry  a  means  rather  than  a 
branch  of  education.  The  experiment  was  successful, 
and  some  of  the  most  delightful  hours  of  my  existence 
have  been  spent  in  marking  the  vigorous  and  cheerful 
expansion  of  mental  power,  when  appealed  to  in  the 
manner  here  described. 

Our  pleasure  was  enhanced  when  we  applied  our 
mathematical  knowledge  to  the  solution  of  physical 
problems.  Many  objects  of  hourly  contact  had  thus  a 
new  interest  and  significance  imparted  to  them.  The 
swing,  the  see-saw,  the  tension  of  the  giant-stride 
ropes,  the  fall  and  rebound  of  the  football,  the  advan- 
tage of  a  small  boy  over  a  large  one  when  turning 
short,  particularly  in  slippery  weather;  all  became 
subjects  of  investigation.  A  lady  stands  before  a 


ON   THE    STUDY   OF   PHYSICS.  301 

looking-glass,  of  her  own  height;  it  was  required  to 
know  how  much  of  the  glass  was  really  useful  to  her? 
We  learned  with  pleasure  the  economic  fact  that  she 
might  dispense  with  the  lower  half  and  see  her  whole 
figure  notwithstanding.  It  was  also  pleasant  to  prove 
by  mathematics,  and  verify  by  experiment,  that  the 
angular  velocity  of  a  reflected  beam  is  twice  that  of 
the  mirror  which  reflects  it.  From  the  hum  of  a  bee 
we  were  able  to  determine  the  number  of  times  the 
insect  flaps  its  wings  in  a  second.  Following  up  our 
researches  upon  the  pendulum,  we  learned  how  Colo- 
nel Sabine  had  made  it  the  means  of  determining  the 
figure  of  the  earth;  and  we  were  also  startled  by  the 
inference  which  the  pendulum  enabled  us  to  draw,  that 
if  the  diurnal  velocity  of  the  earth  were  seventeen 
times  its  present  amount,  the  centrifugal  force  at  the 
equator  would  be  precisely  equal  to  the  force  of  gravi- 
tation, so  that  an  inhabitant  of  those  regions  would 
then  have  the  same  tendency  to  fall  upwards  as  down- 
wards. All  these  things  were  sources  of  wonder  and 
delight  to  us:  and  when  we  remembered  that  we  were 
gifted  with  the  powers  which  had  reached  such  results, 
and  that  the  same  great  field  was  ours  to  work  in,  our 
hopes  arose  that  at  some  future  day  we  might  possibly 
push  the  subject  a  little  further,  and  add  our  own  vic- 
tories to  the  conquests  already  won. 

I  ought  to  apologise  to  you  for  dwelling  so  long 
upon  this  subject;  but  the  days  spent  among  these 
young  philosophers  made  a  deep  impression  on  me.  I 
learned  among  them  something  of  myself  and  of 
human  nature,  and  obtained  some  notion  of  a  teacher's 
vocation.  If  there  be  one  profession  in  England  of 
paramount  importance,  I  believe  it  to  be  that  of  the 
schoolmaster;  and  if  there  be  a  position  where  selfish- 
ness and  incompetence  do  most  serious  mischief,  by 


302  FKAGMENTS    OF    SCIENCE. 

lowering  the  moral  tone  and  exciting  irreverence  and 
cunning  where  reverence  and  noble  truthfulness  ought 
to  be  the  feelings  evoked,  it  is  that  of  the  principal  of 
a  school.  When  a  man  of  enlarged  heart  and  mind 
comes  among  boys, — when  he  allows  his  spirit  to 
stream  through  them,  and  observes  the  operation  of 
his  own  character  evidenced  in  the  elevation  of  theirs, 
— it  would  be  idle  to  talk  of  the  position  of  such  a  man 
being  honourable.  It  is  a  blessed  position.  The  man 
is  a  blessing  to  himself  and  to  all  around  him.  Such 
men,  I  believe,  are  to  be  found  in  England,  and  it  be- 
hoves those  who  busy  themselves  with  the  mechanics 
of  education  at  the  present  day,  to  seek  them  out.  For 
no  matter  what  means  of  culture  may  be  chosen, 
whether  physical  or  philological,  success  must  ever 
mainly  depend  upon  the  amount  of  life,  love,  and 
earnestness,  which  the  teacher  himself  brings  with  him 
to  his  vocation. 

Let  me  again,  and  finally,  remind  you  that  the 
claims  of  that  science  which  finds  in  me  to-day  its  un- 
ripened  advocate,  are  those  of  the  logic  of  Nature  upon 
the  reason  of  her  child — that  its  disciplines,  as  an 
agent  of  culture,  are  based  upon  the  natural  relations 
subsisting  between  Man  and  the  universe  of  which  he 
forms  a  part.  On  the  one  side,  we  have  the  apparently 
lawless  shifting  of  phenomena;  on  the  other  side, 
mind,  which  requires  law  for  its  equilibrium,  and 
through  its  own  indestructible  instincts,  as  well  as 
through  the  teachings  of  experience,  knows  that  these 
phenomena  are  reducible  to  law.  To  chasten  this  ap- 
parent chaos  is  a  problem  which  man  has  set  before 
him.  The  world  was  built  in  order:  and  to  us  are 
trusted  the  will  and  power  to  discern  its  harmonies, 
and  to  make  them  the  lessons  of  our  lives.  From  the 
cradle  to  the  grave  we  are  surrounded  with  objects 


ON   THE    STUDY   OF    PHYSICS.  303 

which  provoke  inquiry.  Descending  for  a  moment 
from  this  high  plea  to  considerations  which  lie  closer 
to  us  as  a  nation — as  a  land  of  gas  and  furnaces,  of 
steam  and  electricity:  as  a  land  which  science,  prac- 
tically applied,  has  made  great  in  peace  and  mighty  in 
war: — I  ask  you  whether  this  '  land  of  old  and  just 
renown '  has  not  a  right  to  expect  from  her  institutions 
a  culture  more  in  accordance  with  her  present  needs 
than  that  supplied  by  declension  and  conjugation? 
And  if  the  tendency  should  be  to  lower  the  estimate 
of  science,  by  regarding  it  exclusively  as  the  instrument 
of  material  prosperity,  let  it  be  the  high  mission  of  our 
universities  to  furnish  the  proper  counterpoise  by 
pointing  out  its  nobler  uses — lifting  the  national  mind 
to  the  contemplation  of  it  as  the  last  development  of 
that '  increasing  purpose '  which  runs  through  the  ages 
and  widens  the  thoughts  of  men. 


XII. 

ON  CRYSTALLINE  AND  SLATY  CLEAVAGE* 

"TTTTEEN  the  student  of  physical  science  has  to  in- 
VV  vestigate  the  character  of  any  natural  force, 
his  first  care  must  be  to  purify  it  from  the  mixture  of 
other  forces,  and  thus  study  its  simple  action.  If,  for 
example,  he  wishes  to  know  how  a  mass  of  liquid  would 
shape  itself  if  at  liberty  to  follow  the  bent  of  its  own 
molecular  forces,  he  must  see  that  these  forces  have 
free  and  undisturbed  exercise.  We  might  perhaps 
refer  him  to  the  dew-drop  for  a  solution  of  the  ques- 
tion; but  here  we  have  to  do,  not  only  with  the  action 
of  the  molecules  of  the  liquid  upon  each  other,  but 
also  with  the  action  of  gravity  upon  the  mass,  which 
pulls  the  drop  downwards  and  elongates  it.  If  he 
would  examine  the  problem  in  its  purity,  he  must  do  as 
Plateau  has  done,  detach  the  liquid  mass  from  the 
action  of  gravity;  he  would  then  find  the  shape  to  be  a 
perfect  sphere.  Natural  processes  come  to  us  in  a 
mixed  manner,  and  to  the  uninstructed  mind  are  a 
mass  of  unintelligible  confusion.  Suppose  half-a- 
dozen  of  the  best  musical  performers  to  be  placed  in 
the  same  room,  each  playing  his  own  instrument  to 
perfection,  but  no  two  playing  the  same  tune;  though 
each  individual  instrument  might  be  a  source  of  per- 
fect music,  still  the  mixture  of  all  would  produce  mere 
noise.  Thus  it  is  with  the  processes  of  nature,  where 
mechanical  and  molecular  laws  intermingle  and  create 

*  From  a  discourse  delivered  in  the  Royal  Institution   of 
Great  Britain,  June  6,  1856. 


SLATES.  305 

apparent  confusion.  Their  mixture  constitutes  what 
may  be  called  the  noise  of  natural  laws,  and  it  is  the 
vocation  of  the  man  of  science  to  resolve  this  noise  into 
its  components,  and  thus  to  detect  the  underlying 
music. 

The  necessity  of  this  detachment  of  one  force  from 
all  other  forces  is  nowhere  more  strikingly  exhibited 
than  in  the  phenomena  of  crystallisation.  Here,  for 
example,  is  a  solution  of  common  sulphate  of  soda  or 
Glauber  salt.  Looking  into  it  mentally,  we  see  the 
molecules  of  that  liquid,  like  disciplined  squadrons 
under  a  governing  eye,  arranging  themselves  into  bat- 
talions, gathering  round  distinct  centres,  and  forming 
themselves  into  solid  masses,  which  after  a  time  assume 
the  visible  shape  of  the  crystal  now  held  in  my  hand. 
I  may,  like  an  ignorant  meddler  wishing  to  hasten 
matters,  introduce  confusion  into  this  order.  This  may 
be  done  by  plunging  a  glass  rod  into  the  vessel;  the 
consequent  action  is  not  the  pure  expression  of  the 
crystalline  forces;  t^e  molecules  rush  together  with 
the  confusion  of  an  unorganised  mob,  and  not  with 
the  steady  accuracy  of  a  disciplined  host.  In  this  mass 
of  bismuth  also  we  have  an  example  of  confused  crys- 
tallisation; but  in  the  crucible  behind  me  a  slower 
process  is  going  on:  here  there  is  an  architect  at  work 
'  who  makes  no  chips,  no  din/  and  who  is  now  building 
the  particles  into  crystals,  similar  in  shape  and  struc- 
ture to  those  beautiful  masses  which  we  see  upon  the 
table.  By  permitting  alum  to  crystallise  in  this  slow 
way,  we  obtain  these  perfect  octahedrons;  by  allowing 
carbonate  of  lime  to  crystallise,  nature  produces  these 
beautiful  rhomboids;  when  silica  crystallises,  we  have 
formed  these  hexagonal  prisms  capped  at  the  ends  by 
pyramids;  by  allowing  saltpetre  to  crystallise  we  have 
these  prismatic  masses,  and  when  carbon  crystallises, 


306  FKAGMENTS    OF    SCIENCE. 

we  have  the  diamond.  If  we  wish  to  obtain  a  perfect 
crystal  we  must  allow  the  molecular  forces  free  play; 
if  the  crystallising  mass  be  permitted  to  rest  upon  a 
surface  it  will  be  flattened,  and  to  prevent  this  a  small 
crystal  must  be  so  suspended  as  to  be  surrounded  on 
all  sides  by  the  liquid,  or,  if  it  rest  upon  the  surface, 
it  must  be  turned  daily  so  as  to  present  all  its  faces  in 
succession  to  the  working  builder. 

In  building  up  crystals  these  little  atomic  bricks 
often  arrange  themselves  into  layers  which  are  perfect- 
ly parallel  to  each  other,  and  which  can  be  separated 
by  mechanical  means;  this  is  called  the  cleavage  of 
the  crystal.  The  crystal  of  sugar  I  hold  in  my  hand 
has  thus  far  escaped  the  solvent  and  abrading  forces 
which  sooner  or  later  determine  the  fate  of  sugar- 
candy.  I  readily  discover  that  it  cleaves  with  peculiar 
facility  in  one  direction.  Again  I  lay  my  knife  upon 
this  piece  of  rocksalt,  and  with  a  blow  cleave  it  in  one 
direction.  Laying  the  knife  at  right  angles  to  its  for- 
mer position,  the  crystal  cleaves  again;  and  finally 
placing  the  knife  at  right  angles  to  the  two  former 
positions,  we  find  a  third  cleavage.  Eocksalt  cleaves 
in  three  directions,  and  the  resulting  solid  is  this  per- 
fect cube,  which  may  be  broken  up  into  any  number 
of  smaller  cubes.  Iceland  spar  also  cleaves  in  three 
directions,  not  at  right  angles,  but  oblique  to  each 
other,  the  resulting  solid  being  a  rhomboid.  In  each 
of  these  cases  the  mass  cleaves  with  equal  facility  in  all 
three  directions.  For  the  sake  of  completeness  I  may 
say  that  many  crystals  cleave  with  unequal  facility 
in  different  directions:  heavy  spar  presents  an  exam- 
ple of  this  kind  of  cleavage. 

Turn  we  now  to  the  consideration  of  some  other 
phenomena  to  which  the  term  cleavage  may  be  applied. 
Beech,  deal,  and  other  woods  cleave  with  facility  along 


SLATES.  307 

the  fibre,  and  this  cleavage  is  most  perfect  when  the 
edge  of  the  axe  is  laid  across  the  rings  which  mark  the 
growth  of  the  tree.  If  you  look  at  this  bundle  of  hay 
severed  from  a  rick,  you  will  see  a  sort  of  cleavage  in  it 
also;  the  stalks  lie  in  horizontal  planes,  and  only  a 
small  force  is  required  to  separate  them  laterally.  But 
we  cannot  regard  the  cleavage  of  the  tree  as  the  same 
in  character  as  that  of  the  hayrick.  In  the  one  case  it 
is  the  molecules  arranging  themselves  according  to 
organic  laws  which  produce  a  cleavable  structure,  in 
the  other  case  the  easy  separation  in  one  direction  is 
due  to  the  mechanical  arrangement  of  the  coarse  sensi- 
ble stalks  of  hay. 

This  sandstone  rock  was  once  a  powder  held  in 
mechanical  suspension  by  water.  The  powder  was  com- 
posed of  two  distinct  parts,  fine  grains  of  sand  and 
small  plates  of  mica.  Imagine  a  wide  strand  covered 
by  a  tide,  or  an  estuary  with  water  which  holds  such 
powder  in  suspension:  how  will  it  sink?  The  rounded 
grains  of  sand  will  reach  the  bottom  first,  because  they 
encounter  least  resistance,  the  mica  afterwards,  and 
when  the  tide  recedes  we  have  the  little  plates  shining 
like  spangles  upon  the  surface  of  the  sand.  Each 
successive  tide  brings  its  charge  of  mixed  powder,  de- 
posits its  duplex  layer  day  after  day,  and  finally  masses 
of  immense  thickness  are  piled  up,  which  by  preserv- 
ing the  alternations  of  sand  and  mica  tell  the  tale  of 
their  formation.  Take  the  sand  and  mica,  mix  them 
together  in  water,  and  allow  them  to  subside;  they 
will  arrange  themselves  in  the  manner  indicated,  and 
by  repeating  the  process  you  can  actually  build  up  a 
mass  which  shall  be  the  exact  counterpart  of  that  pre- 
sented by  nature.  Now  this  structure  cleaves  with 
readiness  along  the  planes  in  which  the  particles  of 
mica  are  strewn.  Specimens  of  such  a  rock  sent  to  me 


308  FKAGMENTS    OF    SCIENCE. 

from  Halifax,  and  other  masses  from  the  quarries  of 
Over  Darwen  in  Lancashire,,  are  here  before  you.  With 
a  hammer  and  chisel  I  can  cleave  them  into  flags; 
indeed  these  flags  are  employed  for  roofing  purposes  in 
the  district  from  which  the  specimens  have  come,  and 
receive  the  name  of  '  slatestone.'  But  you  will  discern 
without  a  word  from  me,  that  this  cleavage  is  not  a 
crystalline  cleavage  any  more  than  that  of  a  hayrick  is. 
It  is  molar,  not  molecular. 

This,  so  far  as  I  am  aware  of,  has  never  been 
imagined,  and  it  has  been  agreed  among  geologists  not 
to  call  such  splitting  as  this  cleavage  at  all,  but  to 
restrict  the  term  to  a  phenomenon  of  a  totally  different 
character. 

Those  who  have  visited  the  slate  quarries  of  Cum- 
berland and  North  Wales  will  have  witnessed  the  phe- 
nomenon to  which  I  refer.  We  have  long  drawn  our 
supply  of  roofing-slates  from  such  quarries;  school- 
boys ciphered  on  these  slates,  they  were  used  for  tomb- 
stones in  churchyards,  and  for  billiard-tables  in  the 
metropolis;  but  not  until  a  comparatively  late  period 
did  men  begin  to  enquire  how  their  wonderful  struc- 
ture was  produced.  What  is  the  agency  which  enables 
us  to  split  Honister  Crag,  or  the  cliffs  of  Snowdon,  into 
laminae  from  crown  to  base?  This  question  is  at  the 
present  moment  one  of  the  great  difficulties  of  geolo- 
gists, and  occupies  their  attention  perhaps  more  than 
any  other.  You  may  wonder  at  this.  Looking  into 
the  quarry  of  Penrhyn,  you  may  be  disposed  to  offer 
the  explanation  I  heard  given  two  years  ago.  '  These 
planes  of  cleavage,'  said  a  friend  who  stood  beside  me 
on  the  quarry's  edge,  '  are  the  planes  of  stratification 
which  have  been  lifted  by  some  convulsion  into  an  al- 
most vertical  position.'  But  this  was  a  mistake,  and 
indeed  here  lies  the  grand  difficulty  of  the  problem. 


SLATES.  309 

The  planes  of  cleavage  stand  in  most  cases  at  a  high 
angle  to  the  bedding.  Thanks  to  Sir  Roderick  Mur- 
chison,  I  am  able  to  place  the  proof  of  this  before  you. 
Here  is  a  specimen  of  slate  in  which  both  the  planes 
of  cleavage  and  of  bedding  are  distinctly  marked,  one 
of  them  making  a  large  angle  with  the  other.  This  is 
common.  The  cleavage  of  slates  then  is  not  a  question 
of  stratification;  what  then  is  its  cause? 

In  an  able  and  elaborate  essay  published  in  1835, 
Prof.  Sedgwick  proposed  the  theory  that  cleavage  is 
due  to  the  action  of  crystalline  or  polar  forces  subse- 
quent to  the  consolidation  of  the  rock.  '  We  may  af- 
firm/ he  says, '  that  no  retreat  of  the  parts,  no  contrac- 
tion of  dimensions  in  passing  to  a  solid  state,  can  ex- 
plain such  phenomena.  They  appear  to  me  only 
resolvable  on  the  supposition  that  crystalline  or  polar 
forces. acted  upon  the  whole  mass  simultaneously  in 
one  direction  and  with  adequate  force/  And  again, 
in  another  place:  '  Crystalline  forces  have  re-arranged 
whole  mountain  masses,  producing  a  beautiful  crystal- 
line cleavage,  passing  alike  through  all  the  strata/  * 
The  utterance  of  such  a  man  struck  deep,  as  it  ought 
to  do,  into  the  minds  of  geologists,  and  at  the  present 
day  there  are  few  who  do  not  entertain  this  view  either 
in  whole  or  in  part.f  The  boldness  of  the  theory,  in- 

*  Transactions  of  the  Geological  Society,  ser.  ii.  vol.  iii.  p.  477. 

t  In  a  letter  to  Sir  Charles  Lyell,  dated  from  the  Cape  of  Good 
Hope  February  20,  1836,  Sir  John  Herschel  writes  as  follows  :— 
'  If  rocks  have  been  so  heated  as  to  allow  of  a  commencement  of 
crystallisation,  that  is  to  say,  if  they  have  been  heated  to  a  point 
at  which  the  particles  can  begin  to  move  amongst  themselves,  or 
at  least  on  their  own  axes,  some  general  law  must  then  determine 
the  position  in  which  these  particles  will  rest  on  cooling.  Proba- 
bly that  position  will  have  some  relation  to  the  direction  in  which 
the  heat  escapes.  Now  when  all  or  a  majority  of  particles  of  the 
same  nature  have  a  general  tendency  to  one  position,  that  must 
of  course  determine  a  cleavage  plane.' 


310  FKAGMENTS    OF    SCIENCE. 

deed,  has,  in  some  cases,  caused  speculation  to  run  riot, 
and  we  have  books  published  on  the  action  of  polar 
forces  and  geologic  magnetism,  which  rather  astonish 
those  who  know  something  about  the  subject.  Ac- 
cording to  this  theory  whole  districts  of  North  "Wales 
and  Cumberland,  mountains  included,  are  neither  more 
nor  less  than  the  parts  of  a  gigantic  crystal.  These 
masses  of  slate  were  originally  fine  mud,  composed  of 
the  broken  and  abraded  particles  of  older  rocks.  They 
contain  silica,  alumina,  potash,  soda,  and  mica  mixed 
mechanically  together.  In  the  course  of  ages  the  mix- 
ture became  consolidated,  and  the  theory  before  us 
assumes  that  a  process  of  crystallisation  afterwards  re- 
arranged the  particles  and  developed  in  it  a  single 
plane  of  cleavage.  Though  a  bold,  and  I  think  inad- 
missible, stretch  of  analogies,  this  hypothesis  has  done 
good  service.  Eight  or  wrong,  a  thoughtfully  uttered 
theory  has  a  dynamic  power  which  operates  against  in- 
tellectual stagnation;  and  even  by  provoking  opposi- 
tion is  eventually  of  service  to  the  cause  of  truth.  It 
would,  however,  have  been  remarkable  if,  among  the 
ranks  of  geologists  themselves,  men  were  not  found 
to  seek  an  explanation  of  slate-cleavage  involving  a  less 
hardy  assumption. 

The  first  step  in  an  enquiry  of  this  kind  is  to  seek 
facts.  This  has  been  done,  and  the  labours  of  Daniel 
Sharpe  (the  late  President  of  the  Geological  Society, 
who,  to  the  loss  of  science  and  the  sorrow  of  all  who 
knew  him,  has  so  suddenly  been  taken  away  from  us), 
Mr.  Henry  Clifton  Sorby,  and  others,  have  furnished 
us  with  a  body  of  facts  associated  with  slaty  cleavage, 
and  having  a  most  important  bearing  upon  the  ques- 
tion. 

Fossil  shells  are  found  in  these  slate-rocks.  I  have 
here  several  specimens  of  such  shells  in  the  actual  rock, 


SLATES.  311 

and  occupying  various  positions  in  regard  to  the  cleav- 
age planes.  They  are  squeezed,  distorted,  and  crushed; 
in  all  cases  the  distortion  leads  to  the  inference  that 
the  rock  which  contains  these  shells  has  been  subjected 
to  enormous  pressure  in  a  direction  at  right  angles  to 
the  planes  of  cleavage.  The  shells  are  all  flattened 
and  spread  out  in  these  planes.  Compare  this  fossil 
trilobite  of  normal  proportions  with  these  others  which 
have  suffered  distortion.  Some  have  lain  across,  some 
along,  and  some  oblique  to  the  cleavage  of  the  slate 
in  which  they  are  found;  but  in  all  cases  the  distortion 
is  such  as  required  for  its  production  a  compressing 
force  acting  at  right  angles  to  the  planes  of  cleavage. 
As  the  trilobites  lay  in  the  mud,  the  jaws  of  a  gigantic 
vice  appear  to  have  closed  upon  them  and  squeezed 
them  into  the  shapes  you  see. 

We  sometimes  find  a  thin  layer  of  coarse  gritty 
material,  between  two  layers  of  finer  rock,  through 
which  and  across  the  gritty  layer  pass  the  planes  of 
lamination.  The  coarse  layer  is  found  bent  by  the 
pressure  into  sinuosities  like  a  contorted  ribbon.  Mr. 
Sorby  has  described  a  striking  case  of  this  kind.  This 
crumpling  can  be  experimentally  imitated;  the  amount 
of  compression  might,  moreover,  be  roughly  estimated 
by  supposing  the  contorted  bed  to  be  stretched  out,  its 
length  measured  and  compared  with  the  shorter  dis- 
tance into  which  it  has  been  squeezed.  We  find  in  this 
way  that  the  yielding  of  the  mass  has  been  consider- 
able. 

Let  me  now  direct  your  attention  to  another  proof 
of  pressure;  you  see  the  varying  colours  which  indicate 
the  bedding  on  this  mass  of  slate.  The  dark  portion  is 
gritty,  being  composed  of  comparatively  coarse  par- 
ticles, which,  owing  to  their  size,  shape  and  gravity, 
sink  first  and  constitute  the  bottom  of  each  layer. 


312  FRAGMENTS    OF    SCIENCE. 

Gradually,  from  bottom  to  top  the  coarseness  dimin- 
ishes, and  near  the  upper  surface  we  have  a  layer  of 
exceedingly  fine  grain.  It  is  the  fine  mud  thus  con- 
solidated from  which  are  derived  the  German  razor- 
stones,  so  much  prized  for  the  sharpening  of  surgical 
instruments.  When  a  bed  is  thin,  the  fine-grain  slate 
is  permitted  to  rest  upon  a  slab  of  the  coarse  slate  in 
contact  with  it;  when  the  fine  bed  is  thick,  it  is  cut 
into  slices  which  are  cemented  to  pieces  of  ordinary 
slate,  and  thus  rendered  stronger.  The  mud  thus  de- 
posited is,  as  might  be  expected,  often  rolled  up  into 
nodular  masses,  carried  forward,  and  deposited  among 
coarser  material  by  the  rivers  from  which  the  slate-mud 
has  subsided.  Here  are  such  nodules  enclosed  in  sand- 
stone. Everybody,  moreover,  who  has  ciphered  upon 
a  school-slate  must  remember  the  whitish-green  spots 
which  sometimes  dotted  the  surface  of  the  slate,  and 
over  which:the  pencil  usually  slid  as  if  the  spots  were 
greasy.  I$ow  these  spots  are  composed  of  the  finer 
mud,  and  they  could  not,  on  account  of  their  fineness, 
lite  the  pencil  like  the  surrounding  gritty  portions  of 
the  slate.  Here  is  a  beautiful  example  of  these  spots: 
you  observe  fhlily  on  the  cleavage  surface,  in  broad 
round  patches.  But  turn  the  slate  edgeways  and  the 
section  of  each  nodule  is  seen  to  be  a  sharp  oval  with 
its  longer  axis  parallel  to  the  cleavage.  This  instruc- 
tive fact  has  been  adduced  by  Mr.  Sorby.  I  have  made 
excursions  to  the  quarries  of  Wales  and  Cumberland, 
and  to  many  of  the  slate  yards  of  London,  and  found 
the  fact  general.  Thus  we  elevate  a  common  experi- 
ence of  our  boyhood  into  evidence  of  the  highest  sig- 
nificance as  regards  a  most  important  geological  prob- 
lem. From  the  magnetic  deportment  of  these  slates, 
I  was  led  to  infer  that  these  spots  contain  a  less  amount 
of  iron  than  the  surrounding  dark  slate.  An  analysis 


SLATES.  313 

was  made  for  me  by  Mr.  Hambly  in  the  laboratory  of 
Dr.  Percy  at  the  School  of  Mines  with  the  following 
result: — 

ANALYSIS  OF  SLATE. 

Dark  SlaJe,  two  analyses, 

1.  Percentage  of  iron 5.85 

2.  "  "  6.13 

Mean    .        .    5.99 

Whitish  Green  Slate. 

1.  Percentage  of  iron 3.24 

2.  "  "  3.12 

Mean    .        .    3.18 

According  to  these  analyses  the  quantity  of  iron  in  the 
dark  slate  immediately  adjacent  to  the  greenish  spot 
is  nearly  double  the  quantity  contained  in  the  spot 
itself.  This  is  about  the  proportion  which  the  mag- 
netic experiments  suggested. 

Let  me  now  remind  you  that  the  facts  brought 
before  you  are  typical — each  is  the  representative  of 
a  class.  We  have  seen  shells  crushed;  the  trilobites 
squeezed,  beds  contorted,  nodules  of  greenish  marl 
flattened;  and  all  these  sources  of  independent  testi- 
mony point  to  one  and  the  same  conclusion,  namely, 
that  date-rocks  have  been  subjected  to  enormous  pres- 
sure in  a  direction  at  right  angles  to  the  planes  of 
cleavage. 

In  reference  to  Mr.  Sorby*s  contorted  bed,  I  have 
said  that  by  supposing  it  to  be  stretched  out  and  its 
length  measured,  it  would  give  us  an  idea  of  the 
amount  of  yielding  of  the  mass  above  and  below  the 
bed.  Such  a  measurement,  however,  would  not  give 
the  exact  amount  of  yielding.  I  hold  in  my  hand  a 
specimen  of  slate  with  its  bedding  marked  upon  it;  the 
lower  portions  of  each  layer  being  composed  of  a  com- 


314  FRAGMENTS    OF    SCIENCE. 

paratively  coarse  gritty  material  something  like  what 
you  may  suppose  the  contorted  bed  to  be  composed  of. 
~Now  in  crossing  these  gritty  portions,  the  cleavage 
turns,  as  if  tending  to  cross  the  bedding  at  another 
angle.  "When  the  pressure  began  to  act,  the  intermedi- 
ate bed,  which  is  not  entirely  unyielding,  suffered 
longitudinal  pressure;  as  it  bent,  the  pressure  became 
gradually  more  transverse,  and  the  direction  of  its 
cleavage  is  exactly  such  as  you  would  infer  from  an 
action  of  this  kind — it  is  neither  quite  across  the  bed, 
nor  yet  in  the  same  direction  as  the  cleavage  of  the 
slate  above  and  below  it,  but  intermediate  between 
both.  Supposing  the  cleavage  to  be  at  right  angles  to 
the  pressure,  thi&  is  the  direction  which  it  ought  to 
take  across  these  more  unyielding  strata. 

Thus  we  have  established  the  concurrence  of  the 
phenomena  of  cleavage  and  pressure — that  they  accom- 
pany each  other;  but  the  question  still  remains,  Is  the 
pressure  sufficient  to  account  for  the  cleavage?  A 
single  geologist,  as  far  as  I  am  aware,  answers  boldly 
in  the  affirmative.  This  geologist  is  Sorby,  who  has 
attacked  the  question  in  the  true  spirit  of  a  physical 
investigator.  Call  to  mind  the  cleavage  of  the  flags 
of  Halifax  and  Over  Darwen,  which  is  caused  by  the 
interposition  of  layers  of  mica  between  the  gritty 
strata.  Mr.  Sorby  finds  plates  of  mica  to  be  also  a  con- 
stituent of  slate-rock.  He  asks  himself,  what  will  be 
the  effect  of  pressure  upon  a  mass  containing  such 
plates  confusedly  mixed  up  in  it?  It  will  be,  he  argues, 
and  he  argues  rightly,  to  place  the  plates  with  their 
flat  surfaces  more  or  less  perpendicular  to  the  direction 
in  which  the  pressure  is  exerted.  He  takes  scales  of 
the  oxide  of. iron,  mixes  them  with  a  fine  powder,  and 
on  squeezing  the  mass  finds  that  the  tendency  of  the 
scales  is  to  set  themselves  at  right  angles  to  the  line  of 


SLATES.  315 

pressure.  Along  the  planes  of  weakness  produced  by 
the  scales  the  mass  cleaves. 

By  tests  of  a  different  character  from  those  applied 
by  Mr.  Sorby,  it  might  be  shown  how  true  his  con- 
clusion is — that  the  effect  of  pressure  on  elongated 
particles,  or  plates,  will  be  such  as  he  describes  it. 
But  while  the  scales  must  be  regarded  as  a  true  cause, 
I  should  not  ascribe  to  them  a  large  share  in  the 
production  of  the  cleavage.  I  believe  that  even  if 
the  plates  of  mica  were  wholly  absent,  the  cleavage  of 
slate-rocks  would  be  much  the  same  as  it  is  at 
present. 

Here  is  a  mass  of  pure  white  wax:  it  contains  no 
mica  particles,  no  scales  of  iron,  or  anything  analogous 
to  them.  Here  is  the  selfsame  substance  submitted  to 
pressure.  I  would  invite  the  attention  of  the  eminent 
geologists  now  before  me  to  the  structure  of  this  wax. 
No  slate  ever  exhibited  so  clean  a  cleavage;  it  splits 
into  laminae  of  surpassing  tenuity,  and  proves  at  a 
single  stroke  that  pressure  is  sufficient  to  produce 
cleavage,  and  that  this  cleavage  is  independent  of  inter- 
mixed plates  or  scales.  I  have  purposely  mixed  this 
wax  with  elongated  particles,  and  am  unable  to  say  at 
the  present  moment  that  the  cleavage  is  sensibly 
affected  by  their  presence — if  anything,  I  should  say 
they  rather  impair  its  fineness  and  clearness  than  pro- 
mote it. 

The  finer  the  slate  is  the  more  perfect  will  be  the 
resemblance  of  its  cleavage  to  that  of  the  wax.  Com- 
pare the  surface  of  the  wax  with  the  surface  of  this 
slate  from  Borrodale  in  Cumberland.  You  have  pre- 
cisely the  same  features  in  both:  you  see  flakes  clinging 
to  the  surfaces  of  each,  which  have  been  partially  torn 
away  in  cleaving.  Let  any  close  observer  compare 
these  two  effects,  he  will,  I  am  persuaded,  be  led  to 


316  FKAGMENTS    OF    SCIENCE. 

the  conclusion  that  they  are  the  product  of  a  common 


cause. 


But  you  will  ask  me  how,  according  to  my  view, 
does  pressure  produce  this  remarkable  result?  This 
may  be  stated  in  a  very  few  words. 

There  is  no  such  thing  in  nature  as  a  body  of  per- 
fectly homogeneous  structure.  I  break  this  clay  which 
seems  so  uniform,  and  find  that  the  fracture  presents 
to  my  eyes  innumerable  surfaces  along  which  it  has 
given  way,  and  it  has  yielded  along  those  surfaces 
because  in  them  the  cohesion  of  the  mass  is  less  than 
elsewhere.  I  break  this  marble,  and  even  this  wax, 
and  observe  the  same  result;  look  at  the  mud  at  the 
bottom  of  a  dried  pond;  look  at  some  of  the  ungrav- 
elled  walks  in  Kensington  Gardens  on  drying  after 
rain, — they  are  cracked  and  split,  and  other  circum- 
stances being  equal,  they  crack  and  split  where  the 
cohesion  is  a  minimum.  Take  then  a  mass  of  partially 
consolidated  mud.  Such  a  mass  is  divided  and  subdi- 
vided by  interior  surfaces  along  which  the  cohesion  is 
comparatively  small.  Penetrate  the  mass  in  idea,  and 
you  will  see  it  composed  of  numberless  irregular  poly- 
hedra  bounded  by  surfaces  of  weak  cohesion.  Imagine 
such  a  mass  subjected  to  pressure, — it  yields  and 
spreads  out  in  the  direction  of  least  resistance;  f  the 

*  I  have  usually  softened  the  wax  by  warming  it,  kneaded  it 
with  the  fingers,  and  pressed  it  between  thick  plates  of  glass  pre- 
viously wetted.  At  the  ordinary  summer  temperature  the  pressed 
wax  is  soft,  and  tears  rather  than  cleaves ;  on  this  account  I  cool 
my  compressed  specimens  in  a  mixture  of  pounded  ice  and  salt, 
and  when  thus  cooled  they  split  cleanly. 

f  It  is  scarcely  necessary  to  say  that  if  the  mass  were  squeezed 
equally  in  all  directions  no  laminated  structure  could  be  produced  ; 
it  must  have  room  to  yield  in  a  lateral  direction.  Mr.  Warren  de 
la  Rue  informs  me  that  he  once  wished  to  obtain  white-lead  in  a 
fine  granular  state,  and  to  accomplish  this  he  first  compressed  it. 


SLATES.  317 

little  polyhedra  become  converted  into  laminae,  sepa- 
rated from  each  other  by  surfaces  of  weak  cohesion, 
and  the  infallible  result  will  be  a  tendency  to  cleave 
at  right  angles  to  the  line  of  pressure. 

Further,  a  mass  of  dried  mud  is  full  of  cavities  and 
fissures.  If  you  break  dried  pipe-clay  you  see  them  in 
great  numbers,  and  there  are  multitudes  of  them  so 
small  that  you  cannot  see  them.  A  flattening  of  these 
cavities  must  take  place  in  squeezed  mud,  and  this 
must  to  some  extent  facilitate  the  cleavage  of  the  mass 
in  the  direction  indicated. 

Although  the  time  at  my  disposal  has  not  permitted 
me  duly  to  develope  these  thoughts,  yet  for  the  last 
twelve  months  the  subject  has  presented  itself  to  me 
almost  daily  under  one  aspect  or  another.  I  have 
never  eaten  a  biscuit  during  this  period  without  re- 
marking the  cleavage  developed  by  the  rolling-pin. 
You  have  only  to  break  a  biscuit  across,  and  to  look  at 
the  fracture,  to  see  the  laminated  structure.  We  have 
here  the  means  of  pushing  the  analogy  further.  I  in- 
vite you  to  compare  the  structure  of  this  slate,  which 
was  subjected  to  a  high  temperature  during  the  confla- 
gration of  Mr.  Scott  Russell's  premises,  with  that  of  a 
biscuit.  Air  or  vapour  within  the  slate  has  caused  it 
to  swell,  and  the  mechanical  structure  it  reveals  is  pre- 
cisely that  of  a  biscuit.  During  these  enquiries  I  have 
received  much  instruction  in  the  manufacture  of  puff- 
paste.  Here  is  some  such  paste  baked  under  my  own 
superintendence.  The  cleavage  of  our  hills  is  acci- 
dental cleavage,  but  this  is  cleavage  with  intention. 
The  volition  of  the  pastrycook  has  entered  into  its 
formation.  It  has  been  his  aim  to  preserve  a  series  of 

The  mould  was  conical,  and  permitted  the  lead  to  spread  out  a 
little  laterally.  The  lamination  was  as  perfect  as  that  of  slate,  and 
it  quite  defeated  him  in  his  effort  to  obtain  a  granular  powder. 


318  FKAGMENTS    OF    SCIENCE. 

surfaces  of  structural  weakness,  along  which  the  dough 
divides  into  layers.  Puff-paste  in  preparation  must 
not  be  handled  too  much;  it  ought,  moreover,  to  be 
rolled  on  a  cold  slab,  to  prevent  the  butter  from  melt- 
ing, and  diffusing  itself,  thus  rendering  the  paste  more 
homogeneous  and  less  liable  to  split.  Puff-paste  is, 
then,  simply  an  exaggerated  case  of  slaty  cleavage. 

The  principle  here  enunciated  is  so  simple  as  to 
be  almost  trivial;  nevertheless,  it  embraces  not  only 
the  cases  mentioned,  but,  if  time  permitted,  it  might 
be  shown  you  that  the  principle  has  a  much  wider 
range  of  application.  When  iron  is  taken  from  the 
puddling  furnace  it  is  more  or  less  spongy,  an  aggre- 
gate in  fact  of  small  nodules:  it  is  at  a  welding  heat, 
and  at  this  temperature  is  submitted  to  the  process  of 
rolling.  Bright  smooth  bars  are  the  result.  But  not- 
withstanding the  high  heat  the  nodules  do  not  per- 
fectly blend  together.  The  process  of  rolling  draws 
them  into  fibres.  Here  is  a  mass  acted  upon  by  dilute 
sulphuric  acid,  which  exhibits  in  a  striking  manner 
this  fibrous  structure.  The  experiment  was  made  by 
my  friend  Dr.  Percy,  without  any  reference  to  the 
question  of  cleavage. 

Break  a  piece  of  ordinary  iron  and  you  have  a 
granular  fracture;  beat  the  iron,  you  elongate  these 
granules,  and  finally  render  the  mass  fibrous.  Here  are 
pieces  of  rails  along  which  the  wheels  of  locomotives 
have  slidden;  the  granules  have  yielded  and  become 
plates.  They  exfoliate  or  come  off  in  leaves;  all  these 
effects  belong,  I  believe,  to  the  great  class  of  phenom- 
ena of  which  slaty  cleavage  forms  the  most  prominent 
example.* 

"We  have  now  reached  the  termination  of  our  task. 

*  For  some  further  observations  on  this  subject  by  Mr.  Sorby 
and  myself,  see  Philosophical  Magazine  for  August,  1856. 


SLATES.  319 

You  have  witnessed  the  phenomena  of  crystallisation, 
and  have  had  placed  before  you  the  facts  which  are 
found  associated  with  the  cleavage  of  slate  rocks.  Such 
facts,  as  expressed  by  Helmholtz,  are  so  many  tele- 
scopes to  our  spiritual  vision,  by  which  we  can  see 
backward  through  the  night  of  antiquity,  and  discern 
the  forces  which  have  been  in  operation  upon  the 
earth's  surface 

Ere  the  lion  roared, 

Or  the  eagle  soared. 

From  evidence  of  the  most  independent  and  trust- 
worthy character,  we  come  to  the  conclusion  that  these 
slaty  masses  have  been  subjected  to  enormous  pressure, 
and  by  the  sure  method  of  experiment  we  have  shown 
— and  this  is  the  only  really  new  point  which  has  been 
brought  before  you — how  the  pressure  is  sufficient  to 
produce  the  cleavage.  Expanding  our  field  of  view, 
we  find  the  self-same  law,  whose  footsteps  we  trace 
amid  the  crags  of  Wales  and  Cumberland,  extending 
into  the  domain  of  the  pastrycook  and  ironfounder; 
nay,  a  wheel  cannot  roll  over  the  half-dried  mud  of  our 
streets  without  revealing  to  us  more  or  less  of  the  fea- 
tures of  this  law.  Let  me  say,  in  conclusion,  that  the 
spirit  in  which  this  problem  has  been  attacked  by 
geologists,  indicates  the  dawning  of  a  new  day  for  their 
science.  The  great  intellects  who  have  laboured  at 
geology,  and  who  have  raised  it  to  its  present  pitch  of 
grandeur,  were  compelled  to  deal  with  the  subject  in 
mass;  they  had  no  time  to  look  after  details.  But  the 
desire  for  more  exact  knowledge  is  increasing;  facts  are 
flowing  in  which,  while  they  leave  untouched  the  in- 
trinsic wonders  of  geology,  are  gradually  supplanting 
by  solid  truths  the  uncertain  speculations  which  beset 
the  subject  in  its  infancy.  Geologists  now  aim  to  imi- 
tate, as  far  as  possible,  the  conditions  of  nature,  and  to 


320  FRAGMENTS    OF    SCIENCE. 

produce  her  results;  they  are  approaching  more  and 
more  to  the  domain  of  physics,  and  I  trust  the  day  will 
soon  come  when  we  shall  interlace  our  friendly  arms 
across  the  common  boundary  of  our  sciences,  and  pur- 
sue our  respective  tasks  in  a  spirit  of  mutual  helpful- 
ness, encouragement  and  goodwill. 

[I  would  now  lay  more  stress  on  the  lateral  yield- 
ing, referred  to  in  the  note  at  the  bottom  of  page  316, 
accompanied  as  it  is  by  tangential  sliding,  than  I  was 
prepared  to  do  when  this  lecture  was  given.  This  slid- 
ing is,  I  think,  the  principal  cause  of  the  planes  of 
weakness,  both  in  pressed  wax  and  slate  rock.  J.  T. 
1871.] 


XIII. 

ON  PARAMAGNETIC  AND  D1AGMAGNETIC 
FORCES* 

THE  notion  of  an  attractive  force,  which  draws 
bodies  towards  the  centre  of  the  earth,  was  en- 
tertained by  Anaxagoras  and  his  pupils,  by  Democritus, 
Pythagoras,  and  Epicurus;  and  the  conjectures  of  these 
ancients  were  renewed  by  Galileo,  Huyghens,  and 
others,  who  stated  that  bodies  attract  each  other  as  a 
magnet  attracts  iron.  Kepler  applied  the  notion  to 
bodies  beyond  the  surface  of  the  earth,  and  affirmed  the 
extension  of  this  force  to  the  most  distant  stars.  Thus 
it  would  appear,  that  in  the  attraction  of  iron  by  a 
magnet  originated  the  conception  of  the  force  of  gravi- 
tation. Nevertheless,  if  we  look  closely  at  the  matter, 
it  will  be  seen  that  the  magnetic  force  possesses  char- 
acters strikingly  distinct  from  those  of  the  force  which 
holds  the  universe  together.  The  theory  of  gravitation 
is,  that  every  particle  of  matter  attracts  every  other 
particle;  in  magnetism  also  we  have  attraction,  but  we 
have  always,  at  the  same  time,  repulsion,  the  final 
effect  being  due  to  the  difference  of  these  two  forces. 
A  body  may  be  intensely  acted  on  by  a  magnet,  and 
still  no  motion  of  translation  will  follow,  if  the  repul- 
sion be  equal  to  the  attraction.  Previous  to  magnetiza- 
tion, a  dipping  needle,  when  its  centre  of  gravity  is 
supported,  stands  accurately  level;  but,  after  magnet- 
ization, one  end  of  it,  in  our  latitude,  is  pulled  towards 

*  Abstract  of  a  discourse  delivered  in  the  Royal  Institution, 
February  1, 1856. 

821 


322  FKAGMENTS    OF    SCIENCE. 

the  north  pole  of  the  earth.  The  needle,  however, 
being  suspended  from  the  arm  of  a  fine  balance,  its 
weight  is  found  unaltered  by  its  magnetization.  In 
like  manner,  when  the  needle  is  permitted  to  float  upon 
a  liquid,  and  thus  to  follow  the  attraction  of  the  north 
magnetic  pole  of  the  earth,  there  is  no  motion  of  the 
mass  towards  that  pole.  The  reason  is  known  to  be, 
that  although  the  marked  end  of  the  needle  is  at- 
tracted by  the  north  pole,  the  unmarked  end  is  re- 
pelled by  an  equal  force,  the  two  equal  and  opposite 
forces  neutralizing  each  other. 

When  the  pole  of  an  ordinary  magnet  is  brought 
to  act  upon  the  swimming  needle,  the  latter  is  at- 
tracted,— the  reason  being  that  the  attracted  end  of 
the  needle  being  nearer  to  the  pole  of  the  magnet 
than  the  repelled  end,  tha  force  of  attraction  is  the 
more  powerful  of  the  two.  In  the  case  of  the  earth, 
its  pole  is  so  distant  that  the  length  of  the  needle  is 
practically  zero.  In  like  manner,  when  a  piece  of  iron 
is  presented  to  a  magnet,  the  nearer  parts  are  attracted, 
while  the  more  distant  parts  are  repelled;  and  because 
the  attracted  portions  are  nearer  to  the  magnet  than 
the  repelled  ones,  we  have  a  balance  in  favour  of  at- 
traction. Here  then  is  the  special  characteristic  of  the 
magnetic  force,  which  distinguishes  it  from  that  of 
gravitation.  The  latter  is  a  simple  unpolar  force,  while 
the  former  is  duplex  or  polar.  Were  gravitation  like 
magnetism,  a  stone  would  no  more  fall  to  the  ground 
than  a  piece  of  iron  towards  the  north  magnetic  pole: 
and  thus,  however  rich  in  consequences  the  supposition 
of  Kepler  and  others  may  have  been,  it  is  clear  that  a 
force  like  that  of  magnetism  would  not  be  able  to  trans- 
act the  business  of  the  universe. 

The  object  of  this  discourse  is  to  enquire  whether 
the  force  of  diamagnetism,  which  manifests  itself  as  a 


PARAMAGNETIC    AND    DIAMAGNETIC    FORCES.    333 

repulsion  of  certain  bodies  by  the  poles  of  a  magnet, 
is  to  be  ranged  as  a  polar  force,  beside  that  of  magnet- 
ism; or  as  an  unpolar  force,  beside  that  of  gravitation. 
When  a  cylinder  of  soft  iron  is  placed  within  a  wire 
helix,  and  surrounded  by  an  electric  current,  the  an- 
tithesis of  its  two  ends,  or,  in  other  words,  its  polar  ex- 
citation, is  at  once  manifested  by  its  action  upon  a 
magnetic  needle;  and  it  may  be  asked  why  a  cylinder 
of  bismuth  may  not  be  substituted  for  the  cylinder  of 
iron,  and  its  state  similarly  examined.  The  reason  is, 
that  the  excitement  of  the  bismuth  is  so  feeble,  that  it 
would  be  quite  masked  by  that  of  the  helix  in  which  it 
is  enclosed;  and  the  problem  that  now  meets  us  is,  so 
to  excite  a  diamagnetic  body  that  the  pure  action  of 
the  body  upon  a  magnetic  needle  may  be  observed, 
unmixed  with  the  action  of  the  body  used  to  excite 
the  diamagnetic. 

How  this  has  been  effected  may  be  illustrated  in  the 
following  manner: — When  through  an  upright  helix 
of  covered  copper  wire,  a  voltaic  current  is  sent,  the 
top  of  the  helix  attracts,  while  its  bottom  repels,  the 
same  pole  of  a  magnetic  needle;  its  central  point,  on 
the  contrary,  is  neutral,  and  exhibits  neither  attraction 
nor  repulsion.  Such  a  helix  is  caused  to  stand  between 

Fio.  10. 


the  two  poles  N*  s'  of  an  astatic  system.*     The  two 
magnets  s  N'  and  s'  N  are  united  by  a  rigid  cross  piece 

*  The  reversal  of  the  poles  of  the  two  magnets,  which  were  of 
the  same  strength,  completely  annulled  the  action  of  the  earth  as 
a  magnet. 


324  FRAGMENTS    OF    SCIENCE. 

at  their  centres,  and  are  suspended  from  the  point  at 
so  that  both  magnets  swing  in  the  same  horizontal 
plane.  It  is  so  arranged  that  the  poles  N'  s'  are  oppo- 
site to  the  central  or  neutral  point  of  the  helix,  so  that 
when  a  current  is  sent  through  the  latter,  the  magnets, 
as  before  explained,  are  unaffected.  Here  then  we  have 
an  excited  helix  which  itself  has  no  action  upon  the 
magnets,  and  we  are  thus  enabled  to  examine  the 
action  of  a  body  placed  within  the  helix  and  excited 
by  it,  undisturbed  by  the  influence  of  the  latter.  The 
helix  being  12  inches  high,  a  cylinder  of  soft  iron  6 
inches  long,  suspended  from  a  string  and  passing  over 
a  pulley,  can  be  raised  or  lowered  within  the  helix. 
When  it  is  so  far  sunk  that  its  lower  end  rests  upon 
the  table,  the  upper  end  finds  itself  between  the  poles 
N'S'  of  the  astatic  system.  The  iron  cylinder  is  thus 
converted  into  a  strong  magnet,  attracting  one  of  the 
poles,  and  repelling  the  other,  and  consequently  de- 
flecting the  entire  astatic  system.  When  the  cylinder 
is  raised  so  that  the  upper  end  is  at  the  level  of  the  top 
of  the  helix,  its  lower  end  comes  between  the  poles 
N'  s';  and  a  deflection  opposed  in  direction  to  the 
former  one  is  the  immediate  consequence.  To  render 
these  deflections  more  easily  visible,  a  mirror  m  is  at- 
tached to  the  system  of  magnets;  a  beam  of  light 
thrown  upon  the  mirror  being  reflected  and  projected 
as  a  bright  disk  against  the  wall.  The  distance  of 
this  image  from  the  mirror  being  considerable,  and  its 
angular  motion  double  that  of  the  latter,  a  very  slight 
motion  of  the  magnet  is  sufficient  to  produce  a  dis- 
placement of  the  image  through  several  yards. 

This  then  is  the  principle  of  the  beautiful  appara- 
tus *  by  which  the  investigation  was  conducted.    It  is 

*  Devised  by  Prof.  W.  Weber,  and  constructed  by  M.  Leyser, 
of  Leipzig. 


PARAMAGNETIC   AND    DIAMAGNETIC    FORCES.    325 

manifest  that  if  a  second  helix  be  placed  between  the 
poles  s  N  with  a  cylinder  within  it,  the  action  upon  the 
astatic  magnet  may  be  exalted.  This  was  the  arrange- 
ment made  use  of  in  the  actual  enquiry.  Thus  to 
intensify  the  feeble  action,  which  it  is  here  our  object 
to  seek,  we  have  in  the  first  place  neutralized  the  action 
of  the  earth  upon  the  magnets,  by  placing  them  astatic- 
ally.  Secondly,  by  making  use  of  two  cylinders,  and 
permitting  them  to  act  simultaneously  on  the  four 
poles  of  the  magnets,  we  have  rendered  the  deflecting 
force  four  times  what  it  would  be,  if  only  a  single  pole 
were  used.  Finally,  the  whole  apparatus  was  enclosed 
in  a  suitable  case  which  protected  the  magnets  from 
air-currents,  and  the  deflections  were  read  off  through 
a  glass  plate  in  the  case,  by  means  of  a  telescope  and 
scale  placed  at  a  considerable  distance  from  the  in- 
strument. 

A  pair  of  bismuth  cylinders  was  first  examined. 
Sending  a  current  through  the  helices,  and  observing 
that  the  magnets  swung  perfectly  free,  it  was  first  ar- 
ranged that  the  bismuth  cylinders  within  the  helices 
had  their  central  or  neutral  points  opposite  to  the  poles 
of  the  magnets.  All  being  at  rest  the  number  on  the 
scale  marked  by  the  cross  wire  of  the  telescope  was 
572.  The  cylinders  were  then  moved,  one  up  the  other 
down,  so  that  two  of  their  ends  were  brought  to  bear 
simultaneously  upon  the  magnetic  poles:  the  magnet 
moved  promptly,  and  after  some  oscillations  *  came  to 
rest  at  the  number  612;  thus  moving  from  a  smaller  to 
a  larger  number.  The  other  two  ends  of  the  bars  were 
next  brought  to  bear  upon  the  magnet:  a  prompt  de- 
flection was  the  consequence,  and  the  final  position  of 
equilibrium  was  526:  the  movement  being  from  a 
larger  to  a  smaller  number.  We  thus  observe  a  mani- 
*  To  lessen  these  a  copper  damper  was  made  use  of. 


326  FRAGMENTS    OF    SCIENCE. 

fest  polar  action  of  the  bismuth  cylinders  ugon  the 
magnet;  one  pair  of  ends  deflecting  it  in  one  direction, 
and  the  other  pair  deflecting  it  in  the  opposite  direc- 
tion. 

Substituting  for  the  cylinders  of  bismuth  thin 
cylinders  of  iron,  of  magnetic  slate,  of  sulphate  of  iron, 
carbonate  of  iron,  protochloride  of  iron,  red  ferrocya- 
nide  of  potassium,  and  other  magnetic  bodies,  it  was 
found  that  when  the  position  of  the  magnetic  cylinders 
was  the  same  as  that  of  the  cylinders  of  bismuth,  the 
deflection  produced  by  the  former  was  always  opposed 
in  direction  to  that  produced  by  the  latter;  and  hence 
the  disposition  of  the  force  in  the  diamagnetic  body 
must  have  been  precisely  antithetical  to  its  disposition 
in  the  magnetic  ones. 

But  it  will  be  urged,  and  indeed  has  been  urged 
against  this  inference,  that  the  deflection  produced  by 
the  bismuth  cylinders  may  be  due  to  induced  currents 
excited  in  the  metal  by  its  motion  within  the  helices. 
In  reply  to  this  objection,  it  may  be  stated,  in  the 
first  place,  that  the  deflection  is  permanent,  and  can- 
not therefore  be  due  to  induced  currents,  which  are 
only  of  momentary  duration.  It  has  also  been  urged 
that  such  experiments  ought  to  be  made  with  other 
metals,  and  with  better  conductors  than  bismuth;  for 
if  due  to  currents  of  induction,  the  better  the  con- 
ductor the  more  exalted  will  be  the  effect.  This  re- 
quirement was  complied  with. 

Cylinders  of  antimony  were  substituted  for  those  of 
bismuth.  This  metal  is  a  better  conductor  of  elec- 
tricity, but  less  strongly  diamagnetic  than  bismuth. 
If  therefore  the  action  referred  to  be  due  to  induced 
currents  we  ought  to  have  it  greater  in  the  case  of  anti- 
mony than  with  bismuth;  but  if  it  springs  from  a  true 
diamagnetic  polarity,  the  action  of  the  bismuth  ought 


PARAMAGNETIC    AND    DIAMAGNETIC    FORCES.    327 

to  exceed  that  of  the  antimony.  Experiment  proves 
this  to  be  the  case.  Hence  the  deflection  produced  by 
these  metals  is  due  to  their  diamagnetic,  and  not  to 
their  conductive  capacity.  Copper  cylinders  were  next 
examined:  here  we  have  a  metal  which  conducts  elec- 
tricity fifty  times  better  than  bismuth,  but  its  diamag- 
netic power  is  nearly  null;  if  the  effects  be  due  to  in- 
duced currents  we  ought  to  have  them  here  in  an 
enormously  exaggerated  degree,  but  no  sensible  deflec- 
tion was  produced  by  the  two  cylinders  of  copper. 

It  has  also  been  proposed  by  the  opponents  of  dia- 
magnetic polarity  to  coat  fragments  of  bismuth  with 
some  insulating  substance,  so  as  to  render  the  forma- 
tion of  induced  currents  impossible,  and  to  test  the 
question  with  cylinders  of  these  fragments.  This  re- 
quirement was  also  fulfilled.  It  is  only  necessary  to 
reduce  the  bismuth  to  powder  and  expose  it  for  a  short 
time  to  the  air  to  cause  the  particles  to  become  so  far 
oxidised  as  to  render  them  perfectly  insulating.  The 
insulating  power  of  the  powder  was  exhibited  experi- 
mentally; nevertheless,  this  powder,  enclosed  in  glass 
tubes,  exhibited  an  action  scarcely  less  powerful  than 
that  of  the  massive  bismuth  cylinders. 

But  the  most  rigid  proof,  a  proof  admitted  to  be 
conclusive  by  those  who  have  denied  the  antithesis  of 
magnetism  and  diamagnetism,  remains  to  be  stated. 
Prisms  of  the  same  heavy  glass  as  that  with  which  the 
diamagnetic  force  was  discovered,  were  substituted  for 
the  metallic  cylinders,  and  their  action  upon  the  mag- 
net was  proved  to  be  precisely  the  same  in  kind  as  that 
of  the  cylinders  of  bismuth.  The  enquiry  was  also 
extended  to  other  insulators:  to  phosphorus,  sulphur, 
nitre,  calcareous  spar,  statuary  marble,  with  the  same 
invariable  result:  each  of  these  substances  was  proved 
to  be  polar,  the  disposition  of  the  force  being  the  same 


328  FKAGMENTS    OF    SCIENCE. 

as  that  of  bismuth  and  the  reverse  of  that  of  iron. 
When  a  bar  of  iron  is  set  erect,  its  lower  end  is  known 
to  be  a  north  pole,  and  its  upper  end  a  south  pole,  in 
virtue  of  the  earth's  induction.  A  marble  statue,  on 
the  contrary,  has  its  feet  a  south  pole,  and  its  head  a 
north  pole,  and  there  is  no  doubt  that  the  same  re- 
mark applies  to  its  living  archetype;  each  man  walking 
over  the  earth's  surface  is  a  true  diamagnet,  with  its 
poles  the  reverse  of  those  of  a  mass  of  magnetic  matter 
of  the  same  shape  and  position. 

An  experiment  of  practical  value,  as  affording  a 
ready  estimate  of  the  different  conductive  powers  of 
two  metals  for  electricity,  was  exhibited  in  the  lecture, 
for  the  purpose  of  proving  experimentally  some  of  the 
statements  made  in  reference  to  this  subject.  A  cube 
of  bismuth  was  suspended  by  a  twisted  string  between 
the  two  poles  of  an  electro-magnet.  The  cube  was 
attached  by  a  short  copper  wire  to  a  little  square  pyra- 
mid, the  base  of  which  was  horizontal,  and  its  sides 
formed  of  four  small  triangular  pieces  of  looking-glass. 
A  beam  of  light  was  suffered  to  fall  upon  this  reflector, 
and  as  the  reflector  followed  the  motion  of  the  cube 
the  images  cast  from  its  sides  followed  each  other  in 
succession,  each  describing  a  circle  about  thirty  feet 
in  diameter.  As  the  velocity  of  rotation  augmented, 
these  images  blended  into  a  continuous  ring  of  light. 
At  a  particular  instant  the  electro-magnet  was  excited, 
currents  were  evolved  in  the  rotating  cube,  and  the 
strength  of  these  currents,  which  increases  with  the 
conductivity  of  the  cube  for  electricity,  was  practically 
estimated  by  the  time  required  to  bring  the  cube  and 
its  associated  mirrors  to  a  state  of  rest.  With  bismuth 
this  time  amounted  to  a  score  of  seconds  or  more:  a 
cube  of  copper,  on  the  contrary,  was  struck  almost 
instantly  motionless  when  the  circuit  was  established. 


XIV. 
PHYSICAL  BASIS  OF  SOLAR  CHEMISTRY* 

OMITTING  all  preface,  attention  was  first  drawn 
to  an  experimental  arrangement  intended  to 
prove  that  gaseous  bodies  radiate  heat  in  different  de- 
grees. Near  a  double  screen  of  polished  tin  was  placed 
an  ordinary  ring  gas-burner,  and  on  this  was  placed 
a  hot  copper  ball,  from  which  a  column  of  heated  air 
ascended.  Behind  the  screen,  but  so  situated  that  no 
ray  from  the  ball  could  reach  the  instrument,  was  an 
excellent  thermo-electric  pile,  connected  by  wires  with 
a  very  delicate  galvanometer.  The  pile  was  known  to 
be  an  instrument  whereby  heat  is  applied  to  the  gen- 
eration of  electric  currents;  the  strength  of  the  current 
being  an  accurate  measure  of  the  quantity  of  the  heat. 
As  long  as  both  faces  of  the  pile  are  at  the  same  tem- 
perature, no  current  is  produced;  but  the  slightest 
difference  in  the  temperature  of  the  two  faces  at  once 
declares  itself  by  the  production  of  a  current,  which, 
when  carried  through  the  galvanometer,  indicates  by 
the  deflection  of  the  needle  both  its  strength  and  its 
direction. 

The  two  faces  of  the  pile  were  in  the  first  instance 
brought  to  the  same  temperature;  the  equilibrium 
being  shown  by  the  needle  of  the  galvanometer  stand- 
ing at  zero.  The  rays  emitted  by  the  current  of  hot  air 
already  referred  to  were  permitted  to  fall  upon  one  of 

*  From  a  discourse  delivered  at  the  Royal  Institution  of  Qreat 
Britain,  June  7,  1861. 


330  FRAGMENTS    OF    SCIENCE. 

the  faces  of  the  pile;  and  an  extremely  slight  move- 
ment of  the  needle  showed  that  the  radiation  from  the 
hot  air,  though  sensible,  was  extremely  feeble.  Con- 
nected with  the  ring-burner  was  a  holder  containing 
oxygen  gas;  and  by  turning  a  cock,  a  stream  of  this 
gas  was  permitted  to  issue  from  the  burner,  strike  the 
copper  ball,  and  ascend  in  a  heated  column  in  front  of 
the  pile.  The  result  was,  that  oxygen  showed  itself, 
as  a  radiator  of  heat,  to  be  quite  as  feeble  as  atmos- 
pheric air. 

A  second  holder  containing  olefiant  gas  was  then 
connected  with  the  ring-burner.  Oxygen  and  air  had 
already  flowed  over  the  ball  and  cooled  it  in  some  de- 
gree. Henee  the  olefiant  gas  laboured  under  a  disad- 
vantage. But  on  permitting  the  gas  to  rise  from  the 
ball,  it  casts  an  amount  of  heat  against  the  adjacent 
face  of  the  pile  sufficient  to  impel  the  needle  of  the 
galvanometer  almost  to  90°.  This  experiment  proved 
the  vast  difference  between  two  equally  invisible  gases 
with  regard  to  their  power  of  emitting  radiant  heat. 

The  converse  experiment  was  now  performed.  The 
thermo-electric  pile  was  removed  and  placed  between 
two  cubes  filled  with  water  kept  in  a  state  of  constant 
ebullition;  and  it  was  so  arranged  that  the  quantities 
of  heat  falling  from  the  cubes  on  the  opposite  faces  of 
the  pile  were  exactly  equal,  thus  neutralising  each 
other.  The  needle  of  the  galvanometer  being  at  zero, 
a  sheet  of  oxygen  gas  was  caused  to  issue  from  a  slit 
between  one  of  the  cubes  and  the  adjacent  face  of  the 
pile.  If  this  sheet  of  gas  possessed  any  sensible  power 
of  intercepting  the  thermal  rays  from  the  cube,  one 
face  of  the  pile  being  deprived  of  the  heat  thus  inter- 
cepted, a  difference  of  temperature  between  its  two 
faces  would  instantly  set  in,  and  the  result  would  be 
declared  by  the  galvanometer.  The  quantity  absorbed 


PHYSICAL    BASIS    OF    SOLAR    CHEMISTRY.   331 

by  the  oxygen  under  those  circumstances  was  too  feeble 
to  affect  the  galvanometer;  the  gas,  in  fact,  proved 
perfectly  transparent  to  the  rays  of  heat.  It  had  but 
a  feeble  power  of  radiation:  it  had  an  equally  feeble 
power  of  absorption. 

The  pile  remaining  in  its  position,  a  sheet  of  ole- 
fiant  gas  was  caused  to  issue  from  the  same  slit  as  that 
through  which  the  oxygen  had  passed.  No  one  present 
could  see  the  gas;  it  was  quite  invisible,  the  light 
went  through  it  as  freely  as  through  oxygen  or  air;  but 
its  effect  "upon  the  thermal  rays  emanating  from  the 
cube  was  what  might  be  expected  from  a  sheet  of 
metal.  A  quantity  so  large  was  cut  off,  that  the  needle 
of  the  galvanometer,  promptly  quitting  the  zero  line, 
moved  with  energy  to  its  stops.  Thus  the  olefiant  gas, 
so  light  and  clear  and  pervious  to  luminous  rays,  was 
proved  to  be  a  most  potent  destroyer  of  the  rays  ema- 
nating from  an  obscure  source.  The  reciprocity  of 
action  established  in  the  case  of  oxygen  comes  out  here; 
the  good  radiator  is  found  by  this  experiment  to  be  the 
good  absorber. 

This  result,  now  exhibited  before  a  public  audience 
for  the  first  time,  was  typical  of  what  had  been  ob- 
tained with  gases  generally.  Going  through  the  entire 
list  of  gases  and  vapours  in  this  way,  we  find  radiation 
and  absorption  to  be  as  rigidly  associated  as  positive 
and  negative  in  electricity,  or  as  north  and  south  polar- 
ity in  magnetism.  So  that  if  we  make  the  number 
which  expresses  the  absorptive  power  the  numerator  of 
a  fraction,  and  that  which  expresses  its  radiative  power 
the  denominator,  the  result  would  be,  that  on  account 
of  the  numerator  and  denominator  varying  in  the  same 
proportion,  the  value  of  that  fraction  would  always 
remain  the  same,  whatever  might  be  the  gas  or  vapour 
experimented  with. 


332  FRAGMENTS    OP    SCIENCE. 

But  why  should  this  reciprocity  exist?  What  is 
the  meaning  of  absorption?  what  is  the  meaning  of 
radiation?  When  you  cast  a  stone  into  still  water, 
rings  of  waves  surround  the  place  where  it  falls;  mo- 
tion is  radiated  on  all  sides  from  the  centre  of  disturb- 
ance. When  a  hammer  strikes  a  bell,  the  latter  vi- 
brates; and  sound,  which  is  nothing  more  than  an 
undulatory  motion  of  the  air,  is  radiated  in  all  direc- 
tions. Modern  philosophy  reduces  light  and  heat  to 
the  same  mechanical  category.  A  luminous  body  is 
one  with  its  atoms  in  a  state  of  vibration;  a'  hot  body 
is  one  with  its  atoms  also  vibrating,  but  at  a  rate  which 
is  incompetent  to  excite  the  sense  of  vision;  and,  as  a 
sounding  body  has  the  air  around  it,  through  which  it 
propagates  its  vibrations,  so  also  the  luminous  or 
heated  body  has  a  medium,  called  ether,  which  accepts 
its  motions  and  carries  them  forward  with  inconceiv- 
able velocity.  Eadiation,  then,  as  regards  both  light 
and  heat,  is  the  transference  of  motion  from  the  vibrat- 
ing body  to  the  ether  in  which  it  swings:  and,  as  in 
the  case  of  sound,  the  motion  imparted  to  the  air  is 
soon  transferred  to  surrounding  objects,  against  which 
the  aerial  undulations  strike,  the  sound  being,  in  tech- 
nical language,  absorbed;  so  also  with  regard  to  light 
and  heat,  absorption  consists  in  the  transference  of 
motion  from  the  agitated  ether  to  the  molecules  of  the 
absorbing  body.  • 

The  simple  atoms  are  found  to  be  bad  radiators; 
the  compound  atoms  good  ones:  and  the  higher  the 
degree  of  complexity  in  the  atomic  grouping,  the  more 
potent,  as  a  general  rule,  is  the  radiation  and  absorp- 
tion. Let  us  get  definite  ideas  here,  however  gross,  and 
purify  them  afterwards  by  the  process  of  abstraction. 
Imagine  our  simple  atoms  swinging  like  single  spheres 
in  the  ether;  they  cannot  create  the  swell  which  a 


PHYSICAL    BASIS    OF    SOLAR   CHEMISTRY.   333 

group  of  them  united  to  form  a  system  can  produce. 
An  oar  runs  freely  edgeways  through  the  water,  and 
imparts  far  less  of  its  motion  to  the  water  than  when 
its  broad  flat  side  is  brought  to  bear  upon  it.  In  our 
present  language  the  oar,  broad  side  vertical,  is  a  good 
radiator;  broad  side  horizontal,  it  is  a  bad  radiator. 
Conversely  the  waves  of  water,  impinging  upon  the  flat 
face  of  the  oar-blade,  will  impart  a  greater  amount  of 
motion  to  it  than  when  impinging  upon  the  edge.  In 
the  position  in  which  the  oar  radiates  well,  it  also  ab- 
sorbs well.  Simple  atoms  glide  through  the  ether  with- 
out much  resistance;  compound  ones  encounter  resist- 
ance, and  hence  yield  up  more  speedily  their  motion 
to  the  ether.  Mix  oxygen  and  nitrogen  mechanically, 
they  absorb  and  radiate  a  certain  amount  of  heat. 
Cause  these  gases  to  combine  chemically  and  form  ni- 
trous oxide,  both  the  absorption  and  radiation  are 
thereby  augmented  hundreds  of  times! 

In  this  way  we  look  with  the  telescope  of  the  in- 
tellect into  atomic  systems,  and  obtain  a  conception  of 
processes  which  the  eye  of  sense  can  never  reach.  But 
gases  and  vapours  possess  a  power  of  choice  as  to  the 
rays  which  they  absorb.  They  single  out  certain 
groups  of  rays  for  destruction,  and  allow  other  groups 
to  pass  unharmed.  This  is  best  illustrated  by  a  famous 
experiment  of  Sir  David  Brewster's,  modified  to  suit 
present  requirements.  Into  a  glass  cylinder,  with  its 
ends  stopped  by  discs  of  plate-glass,  a  small  quantity 
of  nitrous  acid  gas  is  introduced;  the  presence  of  the 
gas  being  indicated  by  its  rich  brown  colour.  The 
beam  from  an  electric  lamp  being  sent  through  two 
prisms  of  bisulphide  of  carbon,  a  spectrum  seven  feet 
long  and  eighteen  inches  wide  is  cast  upon  the  screen. 
Introducing  the  cylinder  containing  the  nitrous  acid 
into  the  path  of  the  beam  as  it  issues  from  the  lamp, 


334  FKAGMENTS    OF    SCIENCE. 

the  splendid  and  continuous  spectrum  becomes  instant- 
ly furrowed  by  numerous  dark  bands,  the  rays  answer- 
ing to  which  are  intercepted  by  the  nitric  gas,  while 
the  light  which  falls  upon  the  intervening  spaces  is 
permitted  to  pass  with  comparative  impunity. 

Here  also  the  principle  of  reciprocity,  as  regards 
radiation  and  absorption,  holds  good;  and  could  we, 
without  otherwise  altering  its  physical  character,  ren- 
der that  nitrous  gas  luminous,  we  should  find  that  the 
very  rays  which  it  absorbs  are  precisely  those  which  it 
would  emit.  When  atmospheric  air  and  other  gases 
are  brought  to  a  state  of  intense  incandescence  by  the 
passage  of  an  electric  spark,  the  spectra  which  we  ob- 
tain from  them  consist  of  a  series  of  bright  bands. 
But  such  spectra  are  produced  with  the  greatest  bril- 
liancy when,  instead  of  ordinary  gases,  we  make  use  of 
metals  heated  so  highly  as  to  volatilise  them.  This  is 
easily  done  by  the  voltaic  current.  A  capsule  of  car- 
bon filled  with  mercury,  which  formed  the  positive 
electrode  of  the  electric  lamp,  has  a  carbon  point 
brought  down  upon  it.  On  separating  the  one  from 
the  other,  a  brilliant  arc  containing  the  mercury  in  a 
volatilised  condition  passes  between  them.  The  spec- 
trum of  this  arc  is  not  continuous  like  that  of  the  solid 
carbon  points,  but  consists  of  a  series  of  vivid  bands, 
each  corresponding  in  colour  to  that  particular  portion 
of  the  spectrum  to  which  its  rays  belong.  Copper  gives 
its  system  of  bands;  zinc  gives  its  system;  and  brass, 
which  is  an  alloy  of  copper  and  zinc,  gives  a  spectrum 
made  up  of  the  bands  belonging  to  both  metals. 

Not  only,  however,  when  metals  are  united  like  zinc 
and  copper  to  form  an  alloy,  is  it  possible  to  obtain 
the  bands  which  belong  to  them.  No  matter  how 
we  may  disguise  the  metal — allowing  it  to  unite  with 
oxygen  to  form  an  oxide,  and  this  again  with  an  acid  to 


PHYSICAL    BASIS    OF    SOLAR   CHEMISTRY.   335 

form  a  salt;  if  the  heat  applied  be  sufficiently  intense, 
the  bands  belonging  to  the  metal  reveal  themselves 
with  perfect  definition.  Into  holes  drilled  in  a  cylinder 
of  retort  carbon,  pure  culinary  salt  is  introduced. 
When  the  carbon  is  made  the  positive  electrode  of  the 
lamp,  the  resultant  spectrum  shows  the  brilliant  yellow 
lines  of  the  metal  sodium.  Similar  experiments  made 
with  the  chlorides  of  strontium,  calcium,  lithium,* 
and  other  metals,  give  the  bands  due  to  the  respective 
metals.  When  different  salts  are  mixed  together,  and 
rammed  into  holes  in  the  carbon;  a  spectrum  is  ob- 
tained which  contains  the  bands  of  them  all. 

The  position  of  these  bright  bands  never  varies, 
and  each  metal  has  its  own  system.  Hence  the  com- 
petent observer  can  infer  from  the  bands  of  the  spec- 
trum the  metals  which  produce  it.  It  is  a  language 
addressed  to  the  eye  instead  of  the  ear;  and  the  cer- 
tainty would  not  be  augmented  if  each  metal  possessed 
the  power  of  audibly  calling  out,  'I  am  here!'  Nor 
is  this  language  affected  by  distance.  If  we  find  that 
the  sun  or  the  stars  give  us  the  bands  of  our  terres- 
trial metals,  it  is  a  declaration  on  the  part  of  these  orbs 
that  such  metals  enter  into  their  composition.  Does 
the  sun  give  us  any  such  intimation?  Does  the  solar 
spectrum  exhibit  bright  lines  which  we  might  com- 
pare with  those  produced  by  our  terrestrial  metals,  and 
prove  either  their  identity  or  difference?  No.  The 
solar  spectrum,  when  closely  examined,  gives  us  a  multi- 
tude of  fine  dark  lines  instead  of  bright  ones.  They 

*  The  vividness  of  the  colours  of  the  lithium  spectrum  is  ex- 
traordinary ;  the  spectrum,  moreover,  contained  a  blue  band  of 
indescribable  splendour.  It  was  thought  by  many,  during  the 
discourse,  that  I  had  mistaken  strontium  for  lithium,  as  this  blue 
band  had  never  before  been  seen.  I  have  obtained  it  many  times 
since;  and  my  friend  Dr.  Miller,  having  kindly  analysed  the  sub- 
stance made  use  of,  pronounces  it  pure  chloride  of  lithium. — J.  T. 


336  FRAGMENTS    OF    SCIENCE. 

were  first  noticed  by  Dr.  Wollaston,  but  were  multi- 
plied and  investigated  with  profound  skill  by  Fraun- 
hofer,  and  named  after  him  Fraunhofer's  lines.  They 
had  been  long  a  standing  puzzle  to  philosophers.  The 
bright  lines  yielded  by  metallic  vapours  had  been  also 
known  to  us  for  years;  but  the  connection  between 
both  classes  of  phenomena  was  wholly  unknown,  until 
Kirchhoff,  with  admirable  acuteness,  revealed  the  se- 
cret, and  placed  it  at  the  same  time  in  our  power  to 
chemically  analyse  the  sun. 

We  have  now  some  difficult  work  before  us.  Hith- 
erto we  have  been  delighted  by  objects  which  addressed 
themselves  as  much  to  our  aesthetic  taste  as  to  our  sci- 
entific faculty;  we  have  ridden  pleasantly  to  the  base 
of  the  final  cone  of  Etna,  and  must  now  dismount  and 
march  through  ashes  and  lava,  if  we  would  enjoy  the 
prospect  from  the  summit.  Our  problem  is  to  connect 
the  dark  lines  of  Fraunhofer  with  the  bright  ones  of 
the  metals.  The  white  beam  of  the  lamp  is  refracted 
in  passing  through  our  two  prisms,  but  its  different 
components  are  refracted  in  different  degrees,  and  thus 
its  colours  are  drawn  apart.  Now  the  colour  depends 
solely  upon  the  rate  of  oscillation  of  the  atoms  of  the 
luminous  body;  red  light  being  produced  by  one  rate, 
blue  light  by  a  much  quicker  rate,  and  the  colours  be- 
tween red  and  blue  by  the  intermediate  rates.  The 
solid  incandescent  coal-points  give  us  a  continuous 
spectrum;  or  in  other  words  they  emit  rays  of  all  possi- 
ble periods  between  the  two  extremes  of  the  spectrum. 
Colour,  as  many  of  you  know,  is  to  light  what  pitch 
is  to  sound.  When  a  violin-player  presses  his  finger  on 
a  string  he  makes  it  shorter  and  tighter,  and  thus, 
causing  it  to  vibrate  more  speedily,  heightens  the  pitch. 
Imagine  such  a  player  to  move  his  fingers  slowly  along 
the  string,  shortening  it  gradually  as  he  draws  his  bow, 


PHYSICAL    BASIS    OF    SOLAR   CHEMISTRY.   337 

the  note  would  rise  in  pitch  by  a  regular  gradation; 
there  would  be  no  gap  intervening  between  note  and 
note.  Here  we  have  the  analogue  to  the  continuous 
spectrum,  whose  colours  insensibly  blend  together 
without  gap  or  interruption,  from  the  red  of  the  lowest 
pitch  to  the  violet  of  the  highest.  But  suppose  the 
player,  instead  of  gradually  shortening  his  string,  to 
press  his  finger  on  a  certain  point,  and  to  sound  the 
corresponding  note;  then  to  pass  on  to  another  point 
more  or  less  distant,  and  sound  its  note;  then  to  an- 
other, and  so  on,  thus  sounding  particular  notes  sepa- 
rated from  each  other  by  gaps  which  correspond  to  the 
intervals  of  the  string  passed  over;  we  should  then  have 
the  exact  analogue  of  a  spectrum  composed  of  separate 
bright  bands  with  intervals  of  darkness  between  them. 
But  this,  though  a  perfectly  true  and  intelligible  anal- 
ogy, is  not  sufficient  for  our  purpose;  we  must  look 
with  the  mind's  eye  at  the  oscillating  atoms  of  the  vola- 
tilised metal.  Figure  these  atoms  as  connected  to- 
gether by  springs  of  a  certain  tension,  which,  if  the 
atoms  are  squeezed  together,  push  them  again  asunder, 
and  if  the  atoms  are  drawn  apart,  pull  them  again 
together,  causing  them,  before  coming  to  rest,  to  quiver 
for  a  certain  time  at  a  certain  definite  rate  determined 
by  the  strength  of  the  spring.  Now  the  volatilised 
metal  which  gives  us  one  bright  band  is  to  be  figured 
as  having  its  atoms  united  by  springs  all  of  the  same 
tension,  its  vibrations  are  all  of  one  kind.  The  metal 
which  gives  us  two  bands  may  be  figured  as  having 
some  of  its  atoms  united  by  springs  of  one  tension,  and 
others  by  springs  of  a  different  tension.  Its  vibrations 
are  of  two  distinct  kinds;  so  also  when  we  have  three 
or  more  bands  we  are  to  figure  as  many  distinct  sets  of 
springs,  each  capable  of  vibrating  in  its  own  particu- 
lar time  and  at  a  different  rate  from  the  others.  If 


338  FKAGMENTS    OF    SCIENCE. 

we  seize  this  idea  definitely,  we  shall  have  no  difficulty 
in  dropping  the  metaphor  of  springs,  and  substituting 
for  it  mentally  the  forces  by  which  the  atoms  act  upon 
each  other.  Having  thus  far  cleared  our  way,  let  us 
make  another  effort  to  advance. 

A  heavy  ivory  ball  is  here  suspended  from  a  string, 
I  blow  against  this  ball;  a  single  puff  of  my  breath 
moves  it  a  little  way  from  its  position  of  rest;  it  swings 
back  towards  me,  and  when  it  reaches  the  limit  of  its 
swing  I  puff  again.  It  now  swings  further;  and  thus 
by  timing  the  puffs  I  can  so  accumulate  their  action  as 
to  produce  oscillations  of  large  amplitude.  The  ivory 
ball  here  has  absorbed  the  motion  which  my  breath 
communicated  to  the  air.  I  now  bring  the  ball  to  rest. 
Suppose,  instead  of  the  breath,  a*  wave  of  air  to  strike 
against  it,  and  that  this  wave  is  followed  by  a  series  of 
others  which  succeed  each  other  exactly  in  the  same 
intervals  as  my  puffs;  it  is  obvious  that  these  waves 
would  communicate  their  motion  to  the  ball  and  cause 
it  to  swing  as  the  puffs  did.  And  it  is  equally  manifest 
that  this  would  not  be  the  case  if  the  impulses  of  the 
waves  were  not  properly  timed;  for  then  the  motion 
imparted  to  the  pendulum  by  one  wave  would  be  neu- 
tralized by  another,  and  there  could  not  be  the  accumu- 
lation of  effect  obtained  when  the  periods  of  the  waves 
correspond  with  the  periods  of  the  pendulum.  So  much 
for  the  particular  impulses  absorbed  by  the  pendulum. 
But  if  such  a  pendulum  set  oscillating  in  air  could  pro- 
duce waves  in  the  air,  it  is  evident  that  the  waves  it 
would  produce  would  be  of  the  same  period  as  those 
whose  motions  it  would  take  up  or  absorb  most  com- 
pletely, if  they  struck  against  it. 

Perhaps  the  most  curious  effect  of  these  timed  im- 
pulses ever  described  was  that  observed  by  a  watch- 
maker, named  Ellicott,  in  the  year  1741.  He  left  two 


PHYSICAL    BASIS    OF    SOLAR    CHEMISTRY.   339 

clocks  leaning  against  the  same  rail;  one  of  them, 
which  we  may  call  A,  was  set  going;  the  other,  B,  not. 
Some  time  afterwards  he  found,  to  his  surprise,  that  B 
was  ticking  also.  The  pendulums  being  of  the  same 
length,  the  shocks  imparted  by  the  ticking  of  A  to  the 
rail  against  which  both  clocks  rested  were  propagated 
to  B,  and  were  so  timed  as  to  set  B  going.  Other  curi- 
ous effects  were  at  the  same  time  observed.  When 
the  pendulums  differed  from  each  other  a  certain 
amount,  A  set  B  going,  but  the  reaction  of  B  stopped 

A.  Then  B  set  A  going,  and  the  reaction  of  A  stopped 

B.  When  the  periods  of  oscillation  were  close  to  each 
other,  but  still  not  quite  alike,  the  clocks  mutually  con- 
trolled each  other,  and  by  a  kind  of  compromise  they 
ticked  in  perfect  unison. 

But  what  has  all  this  to  do  with  our  present  sub- 
ject? The  varied  actions  of  the  universe  are  all  modes 
of  motion;  and  the  vibration  of  a  ray  claims  strict 
brotherhood  with  the  vibrations  of  our  pendulum. 
Suppose  ethereal  waves  striking  upon  atoms  which  os- 
cillate in  the  same  periods  as  the  waves,  the  motion  of 
the  waves  will  be  absorbed  by  the  atoms;  suppose  we 
send  our  beam  of  white  light  through  a  sodium  flame, 
the  atoms  of  that  flame  will  be  chiefly  affected  by  those 
undulations  which  are  synchronous  with  their  own 
periods  of  vibration.  There  will  be  on  the  part  of 
those  particular  rays  a  transference  of  motion  from  the 
agitated  ether  to  the  atoms  of  the  volatilised  metal, 
which,  as  already  denned,  is  absorption. 

The  experiment  justifying  this  conclusion  is  now 
for  the  first  time  to  be  made  before  a  public  audience. 
I  pass  a  beam  through  our  two  prisms,  and  the  spec- 
trum spreads  its  colours  upon  the  screen.  Between  the 
lamp  and  the  prism  I  interpose  a  snapdragon  light. 
Alcohol  and  water  are  here  mixed  with  common  salt, 


340  FKAGMENTS    OF    SCIENCE. 

and  the  metal  dish  that  holds  them  is  heated  by  a 
spirit-lamp.  The  vapour  from  the  mixture  ignites  and 
we  have  a  monochromatic  flame.  Through  this  flame 
the  beam  from  the  lamp  is  now  passing;  and  observe 
the  result  upon  the  spectrum.  You  see  a  shady  band 
cut  out  of  the  yellow, — not  very  dark,  but  sufficiently 
so  to  be  seen  by  everybody  present. 

But  let  me  exalt  this  effect.  Placing  in  front  of 
the  electric  lamp  the  intense  flame  of  a  large  Bunsen's 
burner,  a  platinum  capsule  containing  a  bit  of  sodium 
less  than  a  pea  in  magnitude  is  plunged  into  the  flame. 
The  sodium  soon  volatilises  and  burns  with  brilliant 
incandescence.  The  beam  crosses  the  flame,  and  at  the 
same  time  the  yellow  band  of  the  spectrum  is  clearly 
and  sharply  cut  out,  a  band  of  intense  darkness  occupy- 
ing its  place.  On  withdrawing  the  sodium,  the  bril- 
liant yellow  of  the  spectrum  takes  its  proper  place, 
while  the  reintroduction  of  the  flame  causes  the  band 
to  reappear. 

Let  me  be  more  precise: — The  yellow  colour  of  the 
spectrum  extends  over  a  sensible  space,  blending  on 
one  side  with  the  orange  and  on  the  other  with  the 
green.  The  term  '  yellow  band '  is  therefore  somewhat 
indefinite.  This  vagueness  may  be  entirely  removed. 
By  dipping  the  carbon-point  used  for  the  positive  elec- 
trode into  a  solution  of  common  salt,  and  replacing  it 
in  the  lamp,  the  bright  yellow  band  produced  by  the 
sodium  vapour  stands  out  from  the  spectrum.  When 
the  sodium  flame  is  caused  to  act  upon  the  beam  it  is 
that  particular  yellow  band  that  is  obliterated,  an  in- 
tensely black  streak  occupying  its  place. 

An  additional  step  of  reasoning  leads  to  the  con- 
clusion that  if,  instead  of  the  flame  of  sodium  alone,  we 
were  to  introduce  jnto  the  path  of  the  beam  a  flame  in 
which  lithium,  strontium,  magnesium,  calcium,  &c., 


PHYSICAL    BASIS    OF    SOLAR   CHEMISTRY.   341 

are  in  a  state  of  volatilisation,  each  metallic  vapour 
would  cut  out  a  system  of  bands,  corresponding  ex- 
actly in  position  with  the  bright  bands  of  the  same 
metallic  vapour.  The  light  of  our  electric  lamp  shin- 
ing through  such  a  composite  flame  would  give  us  a 
spectrum  cut  up  by  dark  lines,  exactly  as  the  solar 
spectrum  is  cut  up  by  the  lines  of  Fraunhofer. 

Thus  by  the  combination  of  the  strictest  reasoning 
with  the  most  conclusive  experiment,  we  reach  the 
solution  of  one  of  the  grandest  of  scientific  problems — 
the  constitution  of  the  sun.  The  sun  consists  of  a 
nucleus  surrounded  by  a  flaming  atmosphere.  The 
light  of  the  nucleus  would  give  us  a  continuous  spec- 
trum, like  that  of  our  common  carbon-points;  but  hav- 
ing to  pass  through  the  photosphere,  as  our  beam  had 
to  pass  through  the  flame,  those  rays  of  the  nucleus 
which  the  photosphere  can  itself  emit  are  absorbed, 
and  shaded  spaces,  corresponding  to  the  particular  rays 
absorbed,  occur  in  the  spectrum.  Abolish  the  solar 
nucleus,  and  we  should  have  a  spectrum  showing  a 
bright  line  in  the  place  of  every  dark  line  of  Fraun- 
hofer. These  lines  are  therefore  not  absolutely  dark, 
but  dark  by  an  amount  corresponding  to  the  difference 
between  the  light  of  the  nucleus  intercepted  by  the 
photosphere,  and  the  light  which  issues  from  the  latter. 

The  man  to  whom  we  owe  this  noble  generalisation 
is  Kirchhoff,  Professor  of  Natural  Philosophy  in  the 
University  of  Heidelberg;  *  but,  like  every  other  great 
discovery,  it  is  compounded  of  various  elements.  Mr. 
Talbot  observed  the  bright  lines  in  the  spectra  of 
coloured  flames.  Sixteen  years  ago  Dr.  Miller  gave 
drawings  and  descriptions  of  the  spectra  of  various 
coloured  flames.  Wheatstone,  with  his  accustomed  in- 
genuity, analysed  the  light  of  the  electric  spark,  and 
•  Now  Professor  in  the  University  of  Berlin. 


342  FKAGMENTS    OF    SCIENCE. 

showed  that  the  metals  between  which  the  spark  passed 
determined  the  bright  bands  in  the  spectrum  of  the 
spark.  Masson  published  a  prize  essay  on  these  bands; 
Van  der  Willigen,  and  more  recently  Pliicker,  have 
given  us  beautiful  drawings  of  the  spectra,  obtained 
from  the  discharge  of  Buhmkorffs  coil.  But  none  of 
these  distinguished  men  betrayed  the  least  knowledge 
of  the  connection  between  the  bright  bands  of  the 
metals  and  the  dark  lines  of  the  solar  spectrum.  The 
man  who  came  nearest  to  the  philosophy  of  the  subject 
was  Angstrom.  In  a  paper  translated  from  Poggen- 
dorff s  '  Annalen '  by  myself,  and  published  in  the 
*  Philosophical  Magazine '  for  1855,  he  indicates  that 
the  rays  which  a  body  absorbs  are  precisely  those  which 
it  can  emit  when  rendered  luminous.  In  another  place, 
he  speaks  of  one  of  his  spectra  giving  the  general  im- 
pression of  a  reversal  of  the  solar  spectrum.  Foucault, 
Stokes,  and  Thomson,  have  all  been  very  close  to  the 
discovery;  and,  for  my  own  part,  the  examination  of 
the  radiation  and  absorption  of  heat  by  gases  and 
vapours,  some  of  the  results  of  which  I  placed  before 
you  at  the  commencement  of  this  discourse,  would 
have  led  me  in  1859  to  the  law  on  which  all  KirchhofFs 
speculations  are  founded,  had  not  an  accident  with- 
drawn me  from  the  investigation.  But  KirchhofFs 
claims  are  unaffected  by  these  circumstances.  True, 
much  that  I  have  referred  to  formed  the  necessary 
basis  of  his  discovery;  so  did  the  laws  of  Kepler  fur- 
nish to  Newton  the  basis  of  the  theory  of  gravitation. 
But  what  Kirchhoff  has  done  carries  us  far  beyond  all 
that  had  before  been  accomplished.  He  has  introduced 
the  order  of  law  amid  a  vast  assemblage  of  empirical 
observations,  and  has  ennobled  our  previous  knowl- 
edge by  showing  its  relationship  to  some  of  the  most 
sublime  of  natural  phenomena. 


XV. 

ELEMENTARY  MAGNETISM. 

A  LECTURE  TO  SCHOOLMASTERS. 

~TT"T"E  have  no  reason  to  believe  that  the  sheep  or 
VV  the  dog,  or  indeed  any  of  the  lower  animals, 
feel  an  interest  in  the  laws  by  which  natural  phenom- 
ena are  regulated.  A  herd  may  be  terrified  by  a  thun- 
derstorm; birds  may  go  to  roost,  and  cattle  return  to 
their  stalls,  during  a  solar  eclipse;  but  neither  birds 
nor  cattle,  as  far  as  we  know,  ever  think  of  enquiring 
into  the  causes  of  these  things.  It  is  otherwise  with 
man.  The  presence  of  natural  objects,  the  occurrence 
of  natural  events,  the  varied  appearances  of  the  uni- 
verse in  which  he  dwells  penetrate  beyond  his  organs 
of  sense,  and  appeal  to  an  inner  power  of  which  the 
senses  are  the  mere  instruments  and  excitants.  No  fact 
is  to  him  either  original  or  final.  He  cannot  limit  him- 
self to  the  contemplation  of  it  alone,  but  endeavours 
to  ascertain  its  position  in  a  series  to  which  uniform 
experience  assures  him  it  must  belong.  He  regards  all 
that  he  witnesses  in  the  present  as  the  efflux  and  se- 
quence of  something  that  has  gone  before,  and  as  the 
source  of  a  system  of  events  which  is  to  follow.  The 
notion  of  spontaneity,  by  which  in  his  ruder  state  he 
accounted  for  natural  events,  is  abandoned;  the  idea 
that  nature  is  an  aggregate  of  independent  parts  also 
disappears,  as  the  connection  and  mutual  dependence 
of  physical  powers  become  more  and  more  manifest: 
until  he  is  finally  led  to  regard  Nature  as  an  organic 
whole — as  a  body  each  of  whose  members  sympathises 
28  843 


344  FRAGMENTS    OF    SCIENCE. 

with  the  rest,  changing,  it  is  true,  from  age  to  ago, 
but  changing  without  break  of  continuity  in  the  rela- 
tion of  cause  and  effect. 

The  system  of  things  which  we  call  Nature  is,  how- 
ever, too  vast  and  various  to  be  studied  first-hand  by 
any  single  mind.  As  knowledge  extends  there  is  always 
a  tendency  to  subdivide  the  field  of  investigation.  Its 
various  parts  are  taken  up  by  different  minds,  and  thus 
receive  a  greater  amount  of  attention  than  could  pos- 
sibly be  bestowed  on  them  if  each  investigator  aimed 
at  the  mastery  of  the  whole.  The  centrifugal  form  in 
which  knowledge,  as  a  whole,  advances,  spreading  ever 
wider  on  all  sides,  is  due  in  reality  to  the  exertions  of 
individuals,  each  of  whom  directs  his  efforts,  more  or 
less,  along  a  single  line.  Accepting,  in  many  respects, 
his  culture  from  his  fellow-men — taking  it  from  spoken 
words  or  from  written  books — in  some  one  direction, 
the  student  of  Nature  ought  actually  to  touch  his  work. 
He  may  otherwise  be  a  distributor  of  knowledge,  but 
not  a  creator,  and  he  fails  to  attain  that  vitality  of 
thought,  and  correctness  of  judgment,  which  direct 
and  habitual  contact  with  natural  truth  can  alone  im- 
part. 

One  large  department  of  the  system  of  Nature 
which  forms  the  chief  subject  of  my  own  studies,  and 
to  which  it  is  my  duty  to  call  your  attention  this 
evening,  is  that  of  physics,  or  natural  philosophy.  This 
term  is  large  enough  to  cover  the  study  of  Nature  gen- 
erally, but  it  is  usually  restricted  to  a  department 
which,  perhaps,  lies  closer  to  our  perceptions  than  any 
other.  It  deals  with  the  phenomena  and  laws  of  light 
and  heat — with  the  phenomena  and  laws  of  magnetism 
and  electricity — with  those  of  sound — with  the  pres- 
sures and  motions  of  liquids  and  gases,  whether  at  rest 
or  in  a  state  of  translation  or  of  undulation.  The  sci- 


ELEMENTARY    MAGNETISM.  345 

ence  of  mechanics  is  a  portion  of  natural  philosophy, 
though  at  present  so  large  as  to  need  the  exclusive  at- 
tention of  him  who  would  cultivate  it  profoundly.  As- 
tronomy is  the  application  of  physics  to  the  motions 
of  the  heavenly  bodies,  the  vastness  of  the  field  causing 
it,  however,  to  be  regarded  as  a  department  in  itself. 
In  chemistry  physical  agents  play  important  parts. 
By  heat  and  light  we  cause  atoms  and  molecules  to 
unite  or  to  fall  asunder.  Electricity  exerts  a  similar 
power.  Through  their  ability  to  separate  nutritive 
compounds  into  their  constituents,  the  solar  beams 
build  up  the  whole  vegetable  world,  and  by  it  the  ani- 
mal world.  The  touch  of  the  self-same  beams  causes 
hydrogen  and  chlorine  to  unite  with  sudden  explosion, 
and  to  form  by  their  combination  a  powerful  acid. 
Thus  physics  and  chemistry  intermingle.  Physical 
agents  are,  however,  employed  by  the  chemist  as  a 
means  to  an  end;  while  in  physics  proper  the  laws  and 
phenomena  of  the  agents  themselves,  both  qualitative 
and  quantitative,  are  the  primary  objects  of  attention. 
My  duty  here  to-night  is  to  spend  an  hour  in  telling 
how  this  subject  is  to  be  studied,  and  how  a  knowledge 
of  it  is  to  be  imparted  to  others.  From  the  domain  of 
physics,  which  would  be  unmanageable  as  a  whole,  I 
select  as  a  sample  the  subject  of  magnetism.  I  might 
readily  entertain  you  on  the  present  occasion  with  an 
account  of  what  natural  philosophy  has  accomplished. 
I  might  point  to  those  applications  of  science  of  which 
we  hear  so  much  in  the  newspapers,  and  which  are 
BO  often  mistaken  for  science  itself.  I  might,  of  course, 
ring  changes  on  the  steam-engine  and  the  telegraph, 
the  electrotype  and  the  photograph,  the  medical  appli- 
cations of  physics,  and  the  various  other  inlets  by 
which  scientific  thought  filters  into  practical  life.  That 
v/ould  be  easy  compared  with  the  task  of  informing 


340  FRAGMENTS    OF    SCIENCE. 

you  how  you  are  to  make  the  study  of  physics  the  in- 
strument of  your  pupil's  culture;  how  you  are  to  pos- 
sess its  facts  and  make  them  living  seeds  which  shall 
take  root  and  grow  in  the  mind,  and  not  lie  like  dead 
lumber  in  the  storehouse  of  memory.  This  is  a  task 
much  heavier  than  the  mere  recounting  of  scientific 
achievements;  and  it  is  one  which,  feeling  my  own 
want  of  time  to  execute  it  aright,  I  might  well  hesitate 
to  accept. 

But  let  me  sink  excuses,  and  attack  the  work  before 
me.  First  and  foremost,  then,  I  would  advise  you  to 
get  a  knowledge  of  facts  from  actual  observation. 
Facts  looked  at  directly  are  vital;  when  they  pass  into 
words  half  the  sap  is  taken  out  of  them.  You  wish, 
for  example,  to  get  a  knowledge  of  magnetism;  well, 
provide  yourself  with  a  good  book  on  the  subject,  if 
you  can,  but  do  not  be  content  with  what  the  book 
tells  you;  do  not  be  satisfied  with  its  descriptive  wood- 
cuts; see  the  operations  of  the  force  yourself.  Half 
of  our  book  writers  describe  experiments  which  they 
never  made,  and  their  descriptions  often  lack  both 
force  and  truth;  but,  no  matter  how  clever  or  consci- 
entious they  may  be,  their  written  words  cannot  supply 
the  place  of  actual  observation.  Every  fact  has  numer- 
ous radiations,  which  are  shorn  off  by  the  man  who 
describes  it.  Go,  then,  to  a  philosophical  instrument 
maker,  and  give  a  shilling  or  half  a  crown  for  a  straight 
bar-magnet,  or,  if  you  can  afford  it,  purchase  a  pair  of 
them;  or  get  a  smith  to  cut  a  length  of  ten  inches 
from  a  bar  of  steel  an  inch  wide  and  half  an  inch 
thick;  file  its  ends  smoothly,  harden  it,  and  get  some- 
body like  myself  to  magnetise  it.  Procure  some  darn- 
ing-needles, and  also  a  little  unspun  silk,  which  will 
give  you  a  suspending  fibre  void  of  torsion.  Make  a 
little  loop  of  paper,  or  of  wire,  and  attach  your  fibre 


ELEMENTARY    MAGNETISM.  347 

to  it.  Do  it  neatly.  In  the  loop  place  a  darning- 
needle,  and  bring  the  two  ends  or  poles,  as  they  are 
called,  of  your  bar-magnet  successively  up  to  the  ends 
of  the  needle.  Both  the  poles,  you  find,  attract  both 
ends  of  the  needle.  Replace  the  needle  by  a  bit  of 
annealed  iron  wire;  the  same  effects  ensue.  Suspend 
successively  little  rods  of  lead,  copper,  silver,  brass, 
wood,  glass,  ivory,  or  whalebone;  the  magnet  produces 
no  sensible  effect  upon  any  of  the  substances.  You 
thence  infer  a  special  property  in  the  case  of  steel  and 
iron.  Multiply  your  experiments,  however,  and  you 
will  find  that  some  other  substances,  besides  iron  and 
steel,  are  acted  upon  by  your  magnet.  A  rod  of  the 
metal  nickel,  or  of  the  metal  cobalt,  from  which  the 
blue  colour  used  by  painters  is  derived,  exhibits  pow- 
ers similar  to  those  observed  with  the  iron  and  steel. 

In  studying  the  character  of  the  force  you  may, 
however,  confine  yourself  to  iron  and  steel,  which  are 
always  at  hand.  Make  your  experiments  with  the 
darning-needle  over  and  over  again;  operate  on  both 
ends  of  the  needle;  try  both  ends  of  the  magnet.  Do 
not  think  the  work  dull;  you  are  conversing  with 
Nature,  and  must  acquire  over  her  language  a  certain 
grace  and  mastery,  which  practice  can  alone  impart. 
Let  every  movement  be  made  with  care,  and  avoid 
slovenliness  from  the  outset.  Experiment,  as  I  have 
said,  is  the  language  by  which  we  address  Nature,  and 
through  which  she  sends  her  replies;  in  the  use  of  this 
language  a  lack  of  straightforwardness  is  as  possible, 
and  as  prejudicial,  as  in  the  spoken  language  of  the 
tongue.  If,  therefore,  you  wish  to  become  acquainted 
with  the  truth  of  Nature,  you  must  from  the  first  re- 
solve to  deal  with  her  sincerely. 

Now  remove  your  needle  from  its  loop,  and  draw  it 
from  eye  to  point  along  one  of  the  ends  of  the  magnet; 


348  FRAGMENTS    OF    SCIENCE. 

resuspend  it,  and  repeat  your  former  experiment.  You 
now  find  that  each  extremity  of  the  magnet  attracts 
one  end  of  the  needle,  and  repels  the  other.  The  sim- 
ple attraction  observed  in  the  first  instance,  is  now  re- 
placed by  a  dual  force.  Eepeat  the  experiment  till  you 
have  thoroughly  observed  the  ends  which  attract  and 
those  which  repel  each  other. 

Withdraw  the  magnet  entirely  from  the  vicinity  of 
your  needle,  and  leave  the  latter  freely  suspended  by 
its  fibre.  Shelter  it  as  well  as  you  can  from  currents 
of  air,  and  if  you  have  iron  buttons  on  your  coat,  or  a 
steel  penknife  in  your  pocket,  beware  of  their  action. 
If  you  work  at  night,  beware  of  iron  candlesticks,  or 
of  brass  ones  with  iron  rods  inside.  Freed  from  such 
disturbances,  the  needle  takes  up  a  certain  determinate 
position.  It  sets  its  length  nearly  north  and  south. 
Draw  it  aside  and  let  it  go.  After  several  oscillations 
it  will  again  come  to  the  same  position.  If  you  have 
obtained  your  magnet  from  a  philosophical  instrument 
maker,  you  will  see  a  mark  on  one  of  its  ends.  Sup- 
posing, then,  that  you  drew  your  needle  along  the  end 
thus  marked,  and  that  the  point  of  your  needle  was 
the  last  to  quit  the  magnet,  you  will  find  that  the  point 
turns  to  the  south,  the  eye  of  the  needle  turning  to- 
wards the  north.  Make  sure  of  this,  and  do  not  take 
the  statement  on  my  authority. 

Now  take  a  second  darning-needle  like  the  first, 
and  magnetise  it  in  precisely  the  same  manner:  freely 
suspended  it  also  will  turn  its  eye  to  the  north  and 
its  point  to  the  south.  Your  next  step  is  to  examine 
the  action  of  the  two  needles  which  you  have  thus 
magnetised  upon  each  other. 

Take  one  of  them  in  your  b.and,  and  leave  the 
other  suspended;  bring  the  eye-end  of  the  former  near 
the  eye-end  of  the  latter;  the  suspended  needle  re- 


ELEMENTARY    MAGNETISM.  349 

treats:  it  is  repelled.  Make  the  same  experiment  with 
the  two  points;  you  obtain  the  same  result,  the  sus- 
pended needle  is  repelled.  Now  cause  the  dissimilar 
ends  to  act  on  each  other — you  have  attraction — point 
attracts  eye,  and  eye  attracts  point.  Prove  the  reci- 
procity of  this  action  by  removing  the  suspended 
needle,  and  putting  the  other  in  its  place.  You  ob- 
tain the  same  result.  The  attraction,  then,  is  mutual, 
and  the  repulsion  is  mutual.  You  have  thus  demon- 
strated in  the  clearest  manner  the  fundamental  law  of 
magnetism,  that  like  poles  repel,  and  that  unlike  poles 
attract,  each  other.  You  may  say  that  this  is  all  easily 
understood  without  doing;  but  do  it,  and  your  knowl- 
edge will  not  be  confined  to  what  I  have  uttered  here. 

I  have  said  that  one  end  of  your  bar-magnet  has  a 
mark  upon  it;  lay  several  silk  fibres  together,  so  as  to 
get  sufficient  strength,  or  employ  a  thin  silk  ribbon, 
and  form  a  loop  large  enough  to  hold  your  magnet. 
Suspend  it;  it  turns  its  marked  end  towards  the  north. 
This  marked  end  is  that  which  in  England  is  called  the 
north  pole.  If  a  common  smith  has  made  your  mag- 
net, it  will  be  convenient  to  determine  its  north  pole 
yourself,  and  to  mark  it  with  a  file.  Vary  your  experi- 
ments by  causing  your  magnetised  darning-needle  to 
attract  and  repel  your  large  magnet;  it  is  quite  com- 
petent to  do  so.  In  magnetising  the  needle,  I  have 
supposed  the  point  to  be  the  last  to  quit  the  marked 
end  of  the  magnet;  the  point  of  the  needle  is  a  south 
pole.  The  end  which  last  quits  the  magnet  is  always 
opposed  in  polarity  to  the  end  of  the  magnet  with 
which  it  has  been  last  in  contact. 

You  may  perhaps  learn  all  this  in  a  single  hour; 
but  spend  several  at  it,  if  necessary;  and  remember, 
understanding  it  is  not  sufficient:  you  must  obtain  a 
manual  aptitude  in  addressing  Nature.  If  you  speak 


350  FRAGMENTS    OF    SCIENCE. 

to  your  fellow-man  you  are  not  entitled  to  use  jargon. 
Bad  experiments  are  jargon  addressed  to  Nature,  and 
just  as  much  to  be  .deprecated.  Manual  dexterity  in 
illustrating  the  interaction  of  magnetic  poles  is  of  the 
utmost  importance  at  this  stage  of  your  progress;  and 
you  must  not  neglect  attaining  this  power  over  your 
implements.  As  you  proceed,  moreover,  you  will  be 
tempted  to  do  more  than  I  can  possibly  suggest. 
Thoughts  will  occur  to  you  which  you  will  endeavour 
to  follow  out:  questions  will  arise  which  you  will  try 
to  answer.  The  same  experiment  may  be  twenty 
different  things  to  twenty  people.  Having  witnessed 
the  action  of  pole  on  pole,  through  the  air,  you  will 
perhaps  try  whether  the  magnetic  power  is  not  to  be 
screened  off.  You  use  plates  of  glass,  wood,  slate, 
pasteboard,  or  gutta-percha,  but  find  them  all  pervious 
to  this  wondrous  force.  One  magnetic  pole  acts  upon 
another  through  these  bodies  as  if  they  were  not  pres- 
ent. Should  you  ever  become  a  patentee  for  the  regu- 
lation of  ships'  compasses,  you  will  not  fall,  as  some 
projectors  have  done,  into  the  error  of  screening  off 
the  magnetism  of  the  ship  by  the  interposition  of  such 
substances. 

If  you  wish  to  teach  a  class  you  must  contrive  that 
the  effects  which  you  have  thus  far  witnessed  for  your- 
self shall  be  witnessed  by  twenty  or  thirty  pupils.  And 
here  your  private  ingenuity  must  come  into  play.  You 
will  attach  bits  of  paper  to  your  needles,  so  as  to  ren- 
der their  movements  visible  at  a  distance,  denoting  the 
north  and  south  poles  by  different  colours,  say  green 
and  red.  You  may  also  improve  upon  your  darning- 
needle.  Take  a  strip  of  sheet  steel,  heat  it  to  vivid 
redness  and  plunge  it  into  cold  water.  It  is  thereby 
hardened;  rendered,  in  fact,  almost  as  brittle  as  glass. 
Six  inches  of  this,  magnetised  in  the  manner  of  the 


ELEMENTARY    MAGNETISM.  351 

darning-needle,  will  be  better  able  to  carry  your  paper 
indexes.  Having  secured  such  a  strip,  you  proceed 
thus:— 

Magnetise  a  small  sewing-needle  and  determine  its 
poles;  or,  break  half  an  inch,  or  an  inch,  off  your 
magnetised  darning-needle  and  suspend  it  by  a  fine  silk 
fibre.  The  sewing-needle,  or  the  fragment  of  the  darn- 
ing-needle, is  now  to  be  used  as  a  test-needle,  to  ex- 
amine the  distribution  of  the  magnetism  in  your  strip 
of  steel.  Hold  the  strip  upright  in  your  left  hand, 
and  cause  the  test -needle  to  approach  the  lower  end  of 
your  strip;  one  end  of  the  test-needle  is  attracted,  the 
other  is  repelled.  Raise  your  needle  along  the  strip; 
its  oscillations,  which  at  first  were  quick,  become 
slower;  opposite  the  middle  of  the  strip  they  cease  en- 
tirely; neither  end  of  the  needle  is  attracted;  above 
the  middle  the  test-needle  turns  suddenly  round,  its 
other  end  being  now  attracted.  Go  through  the  ex- 
periment thoroughly:  you  thus  learn  that  the  entire 
lower  half  of  the  strip  attracts  one  end  of  the  needle, 
while  the  entire  upper  half  attracts  the  opposite  end. 
Supposing  the  north  end  of  your  little  needle  to  be 
that  attracted  below,  you  infer  that  the  entire  lower 
half  of  your  magnetised  strip  exhibits  south  magnet- 
ism, while  the  entire  upper  half  exhibits  north  mag- 
netism. So  far,  then,  you  have  determined  the  distri- 
bution of  magnetism  in  your  strip  of  steel. 

You  look  at  this  fact,  you  think  of  it;  in  its  sug- 
gestiveness  the  value  of  an  experiment  chiefly  consists. 
The  thought  naturally  arises:  '  What  will  occur  if  I 
break  my  strip  of  steel  across  in  the  middle?  Shall  I 
obtain  two  magnets  each  possessing  a  single  pole?' 
Try  the  experiment;  break  your  strip  of  steel,  and  test 
each  half  as  you  tested  the  whole.  The  mere  presenta- 
tion of  its  two  ends  in  succession  to  your  test-needle, 


352  FRAGMENTS    OF    SCIENCE. 

suffices  to  show  that  you  have  not  a  magnet  with  a 
single  pole — that  each  half  possesses  two  poles  with  a 
neutral  point  between  them.  And  if  you  again  break 
the  half  into  two  other  halves,  you  will  find  that  each 
quarter  of  the  original  strip  exhibits  precisely  the 
same  magnetic  distribution  as  the  whole  strip.  You 
may  continue  the  breaking  process:  no  matter  how 
small  your  fragment  may  be,  it  still  possesses  two  oppo- 
site poles  and  a  neutral  point  between  them.  Well, 
your  hand  ceases  to  break  where  breaking  becomes  a 
mechanical  impossibility;  but  does  the  mind  stop 
there?  No:  you  follow  the  breaking  process  in  idea 
when  you  can  no  longer  realise  it  in  fact;  your  thoughts 
wander  amid  the  very  atoms  of  your  steel,  and  you 
conclude  that  each  atom  is  a  magnet,  and  that  the 
force  exerted  by  the  strip  of  steel  is  the  mere  summa- 
tion, or  resultant,  of  the  forces  of  its  ultimate  particles. 
Here,  then,  is  an  exhibition  of  power  which  we  can 
call  forth  at  pleasure  or  cause  to  disappear.  We  mag- 
netise our  strip  of  steel  by  drawing  it  along  the  pole  of 
a  magnet;  we  can  demagnetise  it,  or  reverse  its  mag- 
netism, by  properly  drawing  it  along  the  same  pole  in 
the  opposite  direction.  What,  then,  is  the  real  nature 
of  this  wondrous  change?  What  is  it  that  takes  place 
among  the  atoms  of  the  steel  when  the  substance  is 
magnetised?  The  question  leads  us  beyond  the  region 
of  sense,  and  into  that  of  imagination.  This  faculty, 
indeed,  is  the  divining-rod  of  the  man  of  science. 
Not,  however,  an  imagination  which  catches  its  crea- 
tions from  the  air,  but  one  informed  and  inspired  by 
facts;  capable  of  seizing  firmly  on  a  physical  image 
as  a  principle,  of  discerning  its  consequences,  and  of 
devising  means  whereby  these  forecasts  of  thought 
may  be  brought  to  an  experimental  test.  If  such  a 
principle  be  adequate  to  account  for  all  the  phenomena 


ELEMENTARY    MAGNETISM.  353 

— if  from  an  assumed  cause  the  observed  acts  neces- 
sarily follow,  we  call  the  assumption  a  theory,  and, 
once  possessing  it,  we  can  not  only  revive  at  pleasure 
facts  already  known,  but  we  can  predict  others  which 
we  have  never  seen.  Thus,  then,  in  the  prosecution 
of  physical  science,  our  powers  of  observation,  memory, 
imagination,  and  inference,  are  all  drawn  upon.  We 
observe  facts  and  store  them  up;  the  constructive  im- 
agination broods  upon  these  memories,  tries  to  discern 
their  interdependence  and  weave  them  to  an  organic 
whole.  The  theoretic  principle  flashes  or  slowly  dawns 
upon  the  mind;  and  then  the  deductive  faculty  inter- 
poses to  carry  out  the  principle  to  its  logical  conse- 
quences. A  perfect  theory  gives  dominion  over  natural 
facts;  and  even  an  assumption  which  can  only  par- 
tially stand  the  test  of  a  comparison  with  facts,  may 
be  of  eminent  use  in  enabling  us  to  connect  and 
classify  groups  of  phenomena.  The  theory  of  mag- 
netic fluids  is  of  this  latter  character,  and  with  it  we 
must  now  make  ourselves  familiar. 

With  the  view  of  stamping  the  thing  more  firmly 
on  your  minds,  I  will  make  use  of  a  strong  and  vivid 
image.  In  optics,  red  and  green  are  called  comple- 
mentary colours;  their  mixture  produces  white.  Now 
I  ask  you  to  imagine  each  of  these  colours  to  possess 
a  self-repulsive  power;  that  red  repels  red,  that  green 
repels  green;  but  that  red  attracts  green  and  green 
attracts  red,  the  attraction  of  the  dissimilar  colours 
being  equal  to  the  repulsion  of  the  similar  ones. 
Imagine  the  two  colours  mixed  so  as  to  produce  white, 
and  suppose  two  strips  of  wood  painted  with  this 
white;  what  will  be  their  action  upon  each  other? 
Suspend  one  of  them  freely  as  we  suspended  our  darn- 
ing-needle, and  bring  the  other  near  it;  what  will 
occur?  The  red  component  of  the  strip  you  hold  in 


354  FRAGMENTS    OF    SCIENCE. 

your  hand  will  repel  the  red  component  of  your  sus- 
pended strip;  but  then  it  will  attract  the  green,  and, 
the  forces  being  equal,  they  neutralise  each  other.  In 
fact,  the  least  reflection  shows  you  that  the  strips  will 
be  as  indifferent  to  each  other  as  two  unmagnetised 
darning-needles  would  be  under  the  same  circum- 
stances. 

But  suppose,  instead  of  mixing  the  colours,  we 
painted  one  half  of  each  strip  from  centre  to  end  red, 
and  the  other  half  green,  it  is  perfectly  manifest  that 
the  two  strips  would  now  behave  towards  each  other 
exactly  as  our  two  magnetised  darning-needles — the 
red  end  would  repel  the  red  and  attract  the  green,  the 
green  would  repel  the  green  and  attract  the  red;  so 
that,  assuming  two  colours  thus  related  to  each  other, 
we  could  by  their  mixture  produce  the  neutrality  of  an 
unmagnetised  body,  while  by  their  separation  we  could 
produce  the  duality  of  action  of  magnetised  bodies. 

But  you  have  already  anticipated  a  defect  in  my 
conception;  for  if  we  break  one  of  our  strips  of  wood 
in  the  middle  we  have  one  half  entirely  red,  and  the 
other  entirely  green,  and  with  these  it  would  be  im- 
possible to  imitate  the  action  of  our  broken  magnet. 
How,  then,  must  we  modify  our  conception?  We 
must  evidently  suppose  each  molecule  of  tlie  wood 
painted  green  on  one  face  and  red  on  the  opposite  one. 
The  resultant  action  of  all  the  atoms  would  then  ex- 
actly resemble  the  action  of  a  magnet.  Here  also,  if 
the  two  opposite  colours  of  each  atom  could  be  caused 
to  mix  so  as  to  produce  white,  we  should  have,  as  be- 
fore, perfect  neutrality. 

For  these  two  self -repellent  and  mutually  attractive 
colours,  substitute  in  your  minds  two  invisible  self- 
repellent  and  mutually  attractive  fluids,  which  in  or- 
dinary steel  are  mixed  to  form  a  neutral  compound, 


ELEMENTARY    MAGNETISM.  355 

but  which  the  act  of  magnetisation  separates  from  each 
other,  placing  the  opposite  fluids  on  the  opposite  face 
of  each  molecule.  You  have  then  a  perfectly  distinct 
conception  of  the  celebrated  theory  of  magnetic  fluids. 
The  strength  of  the  magnetism  excited  is  supposed  to 
be  proportional  to  the  quantity  of  neutral  fluid  decom- 
posed. According  to  this  theory  nothing  is  actually 
transferred  from  the  exciting  magnet  to  the  excited 
steel.  The  act  of  magnetisation  consists  in  the  forcible 
separation  of  two  fluids  which  existed  in  the  steel  be- 
fore it  was  magnetised,  but  which  then  neutralised 
each  other  by  their  coalescence.  And  if  you  test  your 
magnet,  after  it  has  excited  a  hundred  pieces  of  steel, 
you  will  find  that  it  has  lost  no  force — no  more,  in- 
deed, than  I  should  lose,  had  my  words  such  a  magnetic 
influence  on  your  minds  as  to  excite  in  them  a  strong 
resolve  to  study  natural  philosophy.  I  should  rather 
be  the  gainer  by  my  own  utterance,  and  by  the  reaction 
of  your  fervour.  The  magnet  also  is  the  gainer  by  the 
reaction  of  the  body  which  it  magnetises. 

Look  now  to  your  excited  pieces  of  steel;  figure 
each  molecule  with  its  opposed  fluids  spread  over  its 
opposite  faces.  How  can  this  state  of  things  be  per- 
manent? The  fluids,  by  hypothesis,  attract  each  other; 
what,  then,  keeps  them  apart?  Why  do  they  not 
instantly  rush  together  across  the  equator  of  the  atom, 
and  thus  neutralise  each  other?  To  meet  this  question 
philosophers  have  been  obliged  to  infer  the  existence 
of  a  special  force,  which  holds  the  fluids  asunder.  They 
call  it  coercive  force;  and  it  is  found  that  those  kinds 
of  steel  which  offer  most  resistance  to  being  magnetised 
— which  require  the  greatest  amount  of  '  coercion '  to 
tear  their  fluids  asunder — are  the  very  ones  which  offer 
the  greatest  resistance  to  the  reunion  of  the  fluids, 
after  they  have  been  once  separated.  Such  kinds  of 


356  FRAGMENTS    OF    SCIENCE. 

steel  are  most  suited  to  the  formation  of  permanent 
magnets.  It  is  manifest,  indeed,  that  without  coer- 
cive force  a  permanent  magnet  would  not  be  at  all 
possible. 

Probably  long  before  this  you  will  have  dipped  the 
end  of  your  magnet  among  iron  filings,  and  observed 
how  they  cling  to  it;  or  into  a  nail-box,  and  found 
how  it  drags  the  nails  after  it.  I  know  very  well  that 
if  you  are  not  the  slaves  of  routine,  you  will  have  by 
this  time  done  many  things  that  I  have  not  told  you 
to  do,  and  thus  multiplied  your  experience  beyond 
what  I  have  indicated.  You  are  almost  sure  to  have 
caused  a  bit  of  iron  to  hang  from  the  end  of  your  mag- 
net, and  you  have  probably  succeeded  in  causing  a 
second  bit  to  attach  itself  to  the  first,  a  third  to  the 
second;  until  finally  the  force  has  become  too  feeble 
to  bear  the  weight  of  more.  If  you  have  operated  with 
nails,  you  may  have  observed  that  the  points  and  edges 
hold  together  with  the  greatest  tenacity;  and  that  a  bit 
of  iron  clings  more  firmly  to  the  corner  of  your  magnet 
than  to  one  of  its  flat  surfaces.  In  short,  you  will  in 
all  likelihood  have  enriched  your  experience  in  many 
ways  without  any  special  direction  from  me. 

Well,  the  magnet  attracts  the  nail,  and  the  nail 
attracts  a  second  one.  This  proves  that  the  nail  in 
contact  with  the  magnet  has  had  the  magnetic  quality 
developed  in  it  by  that  contact.  If  it  be  withdrawn 
from  the  magnet  its  power  to  attract  its  fellow  nail 
ceases.  Contact,  however,  is  not  necessary.  A  sheet  of 
glass  or  paper,  or  a  space  of  air,  may  exist  between  the 
magnet  and  the  nail;  the  latter  is  still  magnetised, 
though  not  so  forcibly  as  when  in  actual  contact.  The 
nail  thus  presented  to  the  magnet  is  itself  a  temporary 
magnet.  That  end  which  is  turned  towards  the  mag- 
netic pole  has  the  opposite  magnetism  of  the  pole  which 


ELEMENTARY    MAGNETISM.  357 

excites  it;  the  end  most  remote  from  the  pole  has  the 
same  magnetism  as  the  pole  itself,  and  between  the 
two  poles  the  nail,  like  the  magnet,  possesses  a  mag- 
netic equator. 

Conversant  as  you  now  are  with  the  theory  of  mag- 
netic fluids,  you  have  already,  I  doubt  not,  anticipated 
me  in  imagining  the  exact  condition  of  an  iron  nail 
under  the  influence  of  the  magnet.  You  picture  the 
iron  as  possessing  the  neutral  fluid  in  abundance;  you 
picture  the  magnetic  pole,  when  brought  near,  decom- 
posing the  fluid;  repelling  the  fluid  of  a  like  kind  with 
itself,  and  attracting  the  unlike  fluid;  thus  exciting  in 
the  parts  of  the  iron  nearest  to  itself  the  opposite  po- 
larity. But  the  iron  is  incapable  of  becoming  a  perma- 
nent magnet.  It  only  shows  its  virtue  as  long  as  the 
magnet  acts  upon  it.  What,  then,  does  the  iron  lack 
which  the  steel  possesses?  It  lacks  coercive  force.  Its 
fluids  are  separated  with  ease;  but,  once  the  separating 
cause  is  removed,  they  flow  together  again,  and  neu- 
trality is  restored.  Imagination  must  be  quite  nimble 
in  picturing  these  changes — able  to  see  the  fluids  di- 
viding and  reuniting,  according  as  the  magnet  is 
brought  near  or  withdrawn.  Fixing  a  definite  pole  in 
your  mind,  you  must  picture  the  precise  arrangement 
of  the  two  fluids  with  reference  to  this  pole,  and  be 
able  to  arouse  similar  pictures  in  the  minds  of  your 
pupils.  You  will  cause  them  to  place  magnets  and  iron 
in  various  positions,  and  describe  the  exact  magnetic 
state  of  the  iron  in  each  particular  case.  The  mere 
facts  of  magnetism  will  have  their  interest  immensely 
augmented  by  an  acquaintance  with  the  principles 
whereon  the  facts  depend.  Still,  while  you  use  this 
theory  of  magnetic  fluids  to  track  out  the  phenomena 
and  link  them  together,  you  will  not  forget  to  tell  your 
pupils  that  it  is  to  be  regarded  as  a  symbol  merely, — 


358  FKAGMENTS    OF    SCIENCE. 

a  symbol,  moreover,  which  is  incompetent  to  cover  all 
the  facts,*  but  which  does  good  practical  service  whilst 
we  are  waiting  for  the  actual  truth. 

The  state  of  excitement  into  which  iron  is  thrown 
by  the  influence  of  a  magnet,  is  sometimes  called 
'  magnetisation  by  influence.'  More  commonly,  how- 
ever, the  magnetism  is  said  to  be  '  induced '  in  the 
iron,  and  hence  this  mode  of  magnetising  is  called 
( magnetic  induction/  Now,  there  is  nothing  theo- 
retically perfect  in  Nature:  there  is  no  iron  so  soft  as 
not  to  possess  a  certain  amount  of  coercive  force,  and 
no  steel  so  hard  as  not  to  be  capable,  in  some  degree, 
of  magnetic  induction.  The  quality  of  steel  is  in  some 
measure  possessed  by  iron,  and  the  quality  of  iron  is 
shared  in  some  degree  by  steel.  It  is  in  virtue  of  this 
latter  fact  that  the  unmagnetised  darning-needle  was 
attracted  in  your  first  experiment;  and  from  this  you 
may  at  once  deduce  the  consequence  that,  after  the 
steel  has  been  magnetised,  the  repulsive  action  of  a 
magnet  must  be  always  less  than  its  attractive  action. 
For  the  repulsion  is  opposed  by  the  inductive  action  of 
the  magnet  on  the  steel,  while  the  attraction  is  assisted 
by  the  same  inductive  action.  Make  this  clear  to  your 
minds,  and  verify  it  by  your  experiments.  In  some 
cases  you  can  actually  make  the  attraction  due  to  the 
temporary  magnetism  overbalance  the  repulsion  due  to 
ihe  permanent  magnetism,  and  thus  cause  two  poles  of 
the  same  kind  apparently  to  attract  each  other.  When, 
however,  good  hard  magnets  act  on  each  other  from  a 
sufficient  distance,  the  inductive  action  practically  van- 

*  This  theory  breaks  down  when  applied  to  diamagnetic  bodies 
which  are  repelled  by  magnets.  Like  soft  iron,  such  bodies  are 
thrown  into  a  state  of  temporary  excitement,  in  virtue  of  which 
they  are  repelled ;  but  any  attempt  to  explain  such  a  repulsion 
by  the  decomposition  of  a  fluid  will  demonstrate  its  own  futility. 


ELEMENTARY    MAGNETISM.  359 

ishes,  and  the  repulsion  of  like  poles  is  sensibly  equal 
to  the  attraction  of  unlike  ones. 

I  dwell  thus  long  on  elementary  principles,  because 
they  are  of  the  first  importance,  and  it  is  the  tempta- 
tion of  this  age  of  unhealthy  cramming  to  neglect 
them.  Now  follow  me  a  little  farther.  In  examining 
the  distribution  of  magnetism  in  your  strip  of  steel 
you  raised  the  needle  slowly  from  bottom  to  top,  and 
found  what  we  called  a  neutral  point  at  the  centre. 
Now  does  the  magnet  really  exert  no  influence  on  the 
pole  presented  to  its  centre?  Let  us  see. 

Let  s  N,  fig.  11,  be  our  magnet,  and  let  n  represent 
a  particle  of  north  magnetism  placed  exactly  opposite 
the  middle  of  the  magnet.  Of  course  this  is  an  im- 


I  -IN 


aginary  case,  as  you  can  never  in  reality  thus  detach 
your  north  magnetism  from  its  neighbour.  But  sup- 
posing us  to  have  done  so,  what  would  be  the  action  of 
the  two  poles  of  the  magnet  on  n?  Your  reply  will  of 
course  be  that  the  pole  s  attracts  n  while  the  pole  N 
repels  it.  Let  the  magnitude  and  direction  of  the 
attraction  be  expressed  by  the  line  n  m,  and  the  mag- 
nitude and  direction  of  the  repulsion  by  the  line  n  o. 
Now,  the  particle  n  being  equally  distant  from  s  and  x, 
the  line  n  o,  expressing  the  repulsion,  will  be  equal  to 
m  n,  which  expresses  the  attraction.  Acted  upon  by 
two  such  forces,  the  particle  n  must  evidently  move  in 
the  direction  n  p,  exactly,  midway  between  m  n  and  n  o. 
24 


3  GO  FRAGMENTS    OF    SCIENCE. 

Hence  you  see  that,  although  there  is  no  tendency  of 
the  particle  n  to  move  towards  the  magnetic  equator, 
there  is  a  tendency  on  its  part  to  move  parallel  to  the 
magnet.  If,  instead  of  a  particle  of  north  magnetism, 
we  placed  a  particle  of  south  magnetism  opposite  to 
the  magnetic  equator,  it  would  evidently  be  urged 
along  the  line  n  q;  and  if,  instead  of  two  separate 
particles  of  magnetism,  we  place  a  little  magnetic 
needle,  containing  both  north  and  south  magnetism, 
opposite  the  magnetic  equator,  its  south  pole  being 
urged  along  n  q,  and  its  north  along  n  p,  the  little 
needle  will  be  compelled  to  set  itself  parallel  to  the 
magnet  s  N.  Make  the  experiment,  and  satisfy  your- 
selves that  this  is  a  true  deduction. 

Substitute  for  your  magnetic  needle  a  bit  of  iron 
wire,  devoid  of  permanent  magnetism,  and  it  will  set 
itself  exactly  as  the  needle  does.  Acted  upon  by  the 
magnet,  the  wire,  as  you  know,  becomes  a  magnet  and 
behaves  as  such;  it  will  turn  its  north  pole  towards  p, 
and  south  pole  towards  q,  just  like  the  needle. 

But  supposing  you  shift  the  position  of  your  parti- 
cle of  north  magnetism,  and  bring  it  nearer  to  one  end 


FIG.  12. 


S  I*  "I  N 

of  your  magnet  than  to  the  other;  the  forces  acting 
on  the  particle  are  no  longer  equal;  the  nearest  pole  of 
the  magnet  will  act  more  powerfully  on  the  particle 
than  the  more  distant  one.  Let  s  TS,  fig.  12,  be  the 
magnet,  and  n  the  particle  of  north  magnetism,  in  its 


ELEMENTARY    MAGNETISM.  3d 

new  position.  It  is  repelled  by  N,  and  attracted  by  s. 
Let  the  repulsion  be  represented  in  magnitude  and 
direction  by  the  line  n  o,  and  the  attraction  by  the 
shorter  line  n  w.  The  resultant  of  these  two  forces 
will  be  found  by  completing  the  parallelogram  inn  op, 
and  drawing  its  diagonal  n  p.  Along  n  p,  then,  a 
particle  of  north  magnetism  would  be  urged  by  the 
simultaneous  action  of  s  and  N.  Substituting  a  particle 
of  south  magnetism  for  n,  the  same  reasoning  would 
lead  to  the  conclusion  that  the  particle  would  be  urged 
along  n  q.  If  we  place  at  n  a  short  magnetic  needle, 
its  north  pole  will  be  urged  along  n  p,  its  south  pole 
along  n  q,  the  only  position  possible  to  the  needle, 
thus  acted  on,  being  along  the  line  p  q,  which  is  no 
longer  parallel  to  the  magnet.  Verify  this  deduction 
by  actual  experiment. 

In  this  way  we  might  go  round  the  entire  magnet; 
and,  considering  its  two  poles  as  two  centres  from 
which  the  force  emanates,  we  could,  in  accordance 
with  ordinary  mechanical  principles,  assign  a  definite 
direction  to  the  magnetic  needle  at  every  particular 
place.  And  substituting,  as  before,  a  bit  of  iron  wire 
for  the  magnetic  needle,  the  positions  of  both  will  be 
the  same. 

Now,  I  think,  without  further  preface,  you  will  be 
able  to  comprehend  for  yourselves,  and  explain  to 
others,  one  of  the  most  interesting  effects  in  the  whole 
domain  of  magnetism.  Iron  filings  you  know  are 
particles  of  iron,  irregular  in  shape,  being  longer  in 
some  directions  than  in  others.  For  the  present  ex- 
periment, moreover,  instead  of  the  iron  filings,  very 
small  scraps  of  thin  iron  wire  might  be  employed.  I 
place  a  sheet  of  paper  over  the  magnet;  it  is  all  the 
better  if  the  paper  be  stretched  on  a  wooden  frame,  as 
this  enables  us  to  keep  it  quite  level.  I  scatter  the 


FIG.  13. 


MAGNETIC  LINES  OF  FORCE. 
From  a  Photograph  by  PHOFESSOR  MAYER. 


ELEMENTARY    MAGNETISM.  363 

filings,  or  the  scraps  of  wire,  from  a  sieve  upon  the 
paper,  and  tap  the  latter  gently,  so  as  to  liberate  the 
particles  for  a  moment  from  its  friction.  The  magnet 
acts  on  the  filings  through  the  paper,  and  see  how  it 
arranges  them!  They  embrace  the  magnet  in  a  series 
of  beautiful  curves,  which  are  technically  called  *  mag- 
netic curves,'  or  *  lines  of  magnetic  force.'  Does  the 
meaning  of  these  lines  yet  flash  upon  you?  Set  your 
magnetic  needle,  or  your  suspended  bit  of  wire,  at  any 
point  of  one  of  the  curves,  and  you  will  find  the  direc- 
tion of  the  needle,  or  of  the  wire,  to  be  exactly  that  of 
the  particle  of  iron,  or  of  the  magnetic  curve,  at  that 
point.  Go  round  and  round  the  magnet;  the  direction 
of  your  needle  always  coincides  with  the  direction  of 
the  curve  on  which  it  is  placed.  These,  then,  are  the 
lines  along  which  a  particle  of  south  magnetism,  if  you 
could  detach  it,  would  move  to  the  north  pole,  and  a 
bit  of  north  magnetism  to  the  south  pole.  They  are 
the  lines  along  which  the  decomposition  of  the  neutral 
fluid  takes  place.  In  the  case  of  the  magnetic  needle, 
one  of  its  poles  being  urged  in  one  direction,  and  the 
other  pole  in  the  opposite  direction,  the  needle  must 
necessarily  set  itself  as  a  tangent  to  the  curve.  I  will 
not  seek  to  simplify  this  subject  further.  If  there  be 
anything  obscure  or  confused  or  incomplete  in  my 
statement,  you  ought  now,  by  patient  thought,  to  be 
able  to  clear  away  the  obscurity,  to  reduce  the  con- 
fusion to  order,  and  to  supply  what  is  needed  to  render 
the  explanation  complete.  Do  not  quit  the  subject 
until  you  thoroughly  understand  it;  and  if  you  are 
then  able  to  look  with  your  mind's  eye  at  the  play  of 
forces  around  a  magnet,  and  see  distinctly  the  opera- 
tion of  those  forces  in  the  production  of  the  magnetic 
curves,  the  time  which  we  have  spent  together  will  not 
have  been  spent  in  vain. 


364  FRAGMENTS    OF    SCIENCE. 

In  this  thorough  manner  we  must  master  our  ma- 
terials, reason  upon  them,  and,  by  determined  study, 
attain  to  clearness  of  conception.  Facts  thus  dealt 
with  exercise  an  expansive  force  upon  the  intellect; — 
they  widen  the  mind  to  generalisation.  We  soon 
recognise  a  brotherhood  between  the  larger  phenomena 
of  Nature  and  the  minute  effects  which  we  have  ob- 
served in  our  private  chambers.  Why,  we  enquire, 
does  the  magnetic  needle  set  north  and  south?  Evi- 
dently it  is  compelled  to  do  so  by  the  earth;  the  great 
globe  which  we  inherit  is  itself  a  magnet.  Let  us 
learn  a  little  more  about  it.  By  means  of  a  bit  of 
wax,  or  otherwise,  attach  the  end  of  your  silk  fibre  to 
the  middle  point  of  your  magnetic  needle;  the  needle 
will  thus  be  uninterfered  with  by  the  paper  loop,  and 
will  enjoy  to  some  extent  a  power  of  *  dipping '  its 
point,  or  its  eye,  below  the  horizon.  Lay  your  bar- 
magnet  on  a  table,  and  hold  the  needle  over  the  equator 
of  the  magnet.  The  needle  sets  horizontal.  Move  it 
towards  the  north  end  of  the  magnet;  the  south  end 
of  the  needle  dips,  the  dip  augmenting  as  you  approach 
the  north  pole,  over  which  the  needle,  if  free  to  move, 
will  set  itself  exactly  vertical.  Move  it  back  to  the 
centre,  it  resumes  its  horizontality;  pass  it  on  towards 
the  south  pole,  its  north  end  now  dips,  and  directly 
over  the  south  pole  the  needle  becomes  vertical,  its 
north  end  being  now  turned  downwards.  Thus  we 
learn  that  on  the  one  side  of  the  magnetic  equator  the 
north  end  of  the  needle  dips;  on  the  other  side  the 
south  end  dips,  the  dip  varying  from  nothing  to  90°. 
If  we  go  to  the  equatorial  regions  of  the  earth  with  a 
suitably  suspended  needle  we  shall  find  there  the  posi- 
tion of  the  needle  horizontal.  If  we  sail  north  one 
end  of  the  needle  dips;  if  we  sail  south  the  opposite 
end  dips;  and  over  the  north  or  south  terrestrial 


ELEMENTARY    MAGNETISM.  365 

magnetic  pole  the  needle  sets  vertical.  The  south  mag- 
net pole  has  not  yet  been  found,  but  Sir  James  Ross 
discovered  the  north  magnetic  pole  on  June  1,  1831. 
In  this  manner  we  establish  a  complete  parallelism 
between  the  action  of  the  earth  and  that  of  an  ordinary 
magnet. 

The  terrestrial  magnetic  poles  do  not  coincide  with 
the  geographical  ones;  nor  does  the  earth's  magnetic 
equator  quite  coincide  with  the  geographical  equator. 
The  direction  of  the  magnetic  needle  in  London,  which 
is  called  the  magnetic  meridian,  encloses  an  angle  of 
24°  with  the  astronomical  meridian,  this  angle  being 
called  the  Declination  of  the  needle  for  London.  The 
north  pole  of  the  needle  now  lies  to  the  west  of  the 
true  meridian;  the  declination  is  westerly.  In  the  year 
1660,  however,  the  declination  was  nothing,  while  be- 
fore that  time  it  was  easterly.  All  this  proves  that 
the  earth's  magnetic  constituents  are  gradually  chang- 
ing their  distribution.  This  change  is  very  slow:  it  is 
therefore  called  the  secular  change,  and  the  observation 
of  it  has  not  yet  extended  over  a  sufficient  period  to 
enable  us  to  guess,  even  approximately,  at  its  laws. 

Having  thus  discovered,  to  some  extent,  the  secret 
of  the  earth's  magnetic  power,  we  can  turn  it  to  ac- 
count. In  the  line  of  '  dip '  I  hold  a  poker  formed  of 
good  soft  iron.  The  earth,  acting  as  a  magnet,  is  at 
this  moment  constraining  the  two  fluids  of  the  poker 
to  separate,  making  the  lower  end  of  the  poker  a  north 
pole,  and  the  upper  end  a  south  pole.  Mark  the  experi- 
ment: When  the  knob  is  uppermost,  it  attracts  the 
north  end  of  a  magnetic  needle;  when  undermost  it 
attracts  the  south  end  of  a  magnetic  needle.  With 
such  a  poker  repeat  this  experiment  and  satisfy  your- 
selves that  the  fluids  shift  their  position  according  to 
the  manner  in  which  the  poker  is  presented  to  the 


366  FRAGMENTS    OF    SCIENCE. 

earth.  It  has  already  been  stated  that  the  softest  iron 
possesses  a  certain  amount  of  coercive  force.  The 
earth,  at  this  moment,  finds  in  this  force  an  antagonist 
which  opposes  the  decomposition  of  the  neutral  fluid. 
The  component  fluids  may  be  figured  as  meeting  an 
amount  of  friction,  or  possessing  an  amount  of  ad- 
hesion, which  prevents  them  from  gliding  over  the 
molecules  of  the  poker.  Can  we  assist  the  earth  in  this 
case?  If  we  wish  to  remove  the  residue  of  a  powder 
from  the  interior  surface  of  a  glass  to  which  the  powder 
clings,  we  invert  the  glass,  tap  it,  loosen  the  hold  of 
the  powder,  and  thus  enable  the  force  of  gravity  to  pull 
it  down.  So  also  by  tapping  the  end  of  the  poker  we 
loosen  the  adhesion  of  the  magnetic  fluids  to  the  mole- 
cules and  enable  the  earth  to  pull  them  apart.  But, 
what  is  the  consequence?  The  portion  of  fluid  which 
has  been  thus  forcibly  dragged  over  the  molecules 
refuses  to  return  when  the  poker  has  been  removed 
from  the  line  of  dip;  the  iron,  as  you  see,  has  become 
a  permanent  magnet.  By  reversing  its  position  and 
tapping  it  again  we  reverse  its  magnetism.  A  thought- 
ful and  competent  teacher  will  know  how  to  place 
these  remarkable  facts  before  his  pupils  in  a  manner 
which  will  excite  their  interest.  By  the  use  of  sensible 
images,  more  or  less  gross,  he  will  first  give  those  whom 
he  teaches  definite  conceptions,  purifying  these  con- 
ceptions afterwards,  as  the  minds  of  his  pupils  become 
more  capable  of  abstraction.  By  thus  giving  them  a 
distinct  substratum  for  their  reasonings,  he  will  con- 
fer upon  his  pupils  a  profit  and  a  joy  which  the  mere 
exhibition  of  facts  without  principles,  or  the  appeal  to 
the  bodily  senses  and  the  power  of  memory  alone,  could 
never  inspire. 


ELEMENTARY   MAGNETISM.  367 

As  an  expansion  of  the  note  tit  p.  357,  the  following  extract 
may  find  a  place  here : — 

'  It  is  well  known  that  a  voltaic  current  exerts  an  attractive 
force  upon  a  second  current,  flowing  in  the  same  direction  ;  and 
that  when  the  directions  are  opposed  to  each  other  the  force  ex- 
erted is  a  repulsive  one.  By  coiling  wires  into  spirals,  Ampere 
was  enabled  to  make  them  produce  all  the  phenomena  of  attrac- 
tion and  repulsion  exhibited  by  magnets,  and  from  this  it  was  but 
a  step  to  his  celebrated  theory  of  molecular  currents.  He  sup- 
posed the  molecules  of  a  magnetic  body  to  be  surrounded  by  such 
currents,  which,  however,  in  the  natural  state  of  the  body  mutu- 
ally neutralised  each  other,  on  account  of  their  confused  grouping. 
The  act  of  magnetisation  he  supposed  to  consist  in  setting  these 
molecular  currents  parallel  to  each  other ;  and,  starting  from  this 
principle,  he  reduced  all  the  phenomena  of  magnetism  to  the 
mutual  action  of  electric  currents. 

'  If  we  reflect  upon  the  experiments  recorded  in  the  foregoing 
pages  from  first  to  last,  we  can  hardly  fail  to  be  convinced  that 
diamagnetic  bodies  operated  on  by  magnetic  forces  possess  a  po- 
larity "  the  same  in  kind  as,  but  the  reverse  in  direction  of,  that 
acquired  by  magnetic  bodies."  But  if  this  be  the  case,  how  are 
we  to  conceive  the  physical  mechanism  of  this  polarity  I  Accord- 
ing to  Coulomb's  and  Poisson's  theory,  the  act  of  magnetisation 
consists  in  the  decomposition  of  a  neutral  magnetic  fluid  ;  the 
north  pole  of  a  magnet,  for  example,  possesses  an  attraction  for 
the  south  fluid  of  a  piece  of  soft  iron  submitted  to  its  influence, 
draws  the  said  fluid  towards  it,  and  with  it  the  material  particles 
with  which  the  fluid  is  associated.  To  account  for  diumagnetic 
phenomena  this  theory  seems  to  fail  altogether ;  according  to  it, 
indeed,  the  oft-used  phrase,  "a  north  pole  exciting  a  north  pole, 
and  a  south  pole  a  south  pole,"  involves  a  contradiction.  For  if 
the  north  fluid  be  supposed  to  be  attracted  towards  the  influencing 
north  pole,  it  is  absurd  to  suppose  that  its  presence  there  could 
produce  repulsion.  The  theory  of  Ampere  is  equally  at  a  loss  to 
explain  diamagnetic  action ;  for  if  we  suppose  the  particles  of 
bismuth  surrounded  by  molecular  currents,  then,  according  to  all 
that  is  known  of  electro-dynamic  laws,  those  currents  would.  set 
themselves  parallel  to,  and  in  the  same  direction  as,  those  of  tho 
magnet,  and  hence  attraction,  and  not  repulsion,  would  be  tho 
result.  The  fact,  however,  of  this  not  being  the  case,  proves  that 
these  molecular  currents  are  not  the  mechanism  by  which  dia- 
magnotic  induction  is  effected.  The  consciousness  of  this,  1 
doubt  not,  drove  M.  Weber  to  the  assumption  that  tho  phenomena 


368  FRAGMENTS    OF    SCIENCE. 

of  diamagnetism  are  produced  by  molecular  currents,  not  directed, 
but  actually  excited  in  the  bismuth  by  the  magnet.  Such  induced 
currents  would,  according  to  known  laws,  have  a  direction  op- 
posed to  those  of  the  inducing  magnet,  and  hence  would  produce 
the  phenomena  of  repulsion.  To  carry  out  the  assumption  here 
made,  M.  Weber  is  obliged  to  suppose  that  the  molecules  of  dia- 
magnetic  bodies  are  surrounded  by  channels,  in  which  the  induced 
molecular  currents,  once  excited,  continue  to  flow  without  resist- 
ance.' * — Diamagnetism  and  Magne-crystallic  Action,  p.  136-7. 


*  In  assuming  these  non-resisting  channels  M.  Weber,  it  must 
be  admitted,  did  not  go  beyond  the  assumptions  of  Ampere. 


XVI. 

0-^  FORCE* 

A  SPHEEE  of  lead  was  suspended  at  a  height  of  16 
JL\.  feet  above  the  theatre  floor  of  the  Royal  Institu- 
tion. It  was  liberated,  and  fell  by  gravity.  That 
weight  required  a  second  to  fall  to  the  floor  from  that 
elevation;  and  the  instant  before  it  touched  the  floor, 
it  had  a  velocity  of  32  feet  a  second.  That  is  to  say, 
if  at  that  instant  the  earth  were  annihilated,  and  its 
attraction  annulled,  the  weight  would  proceed  through 
space  at  the  uniform  velocity  of  32  feet  a  second. 

If  instead  of  being  pulled  downward  by  gravity, 
the  weight  be  cast  upward  in  opposition  to  gravity, 
then,  to  reach  a  height  of  16  feet  it  must  start  with  a 
velocity  of  32  feet  a  second.  This  velocity  imparted 
to  the  weight  by  the  human  hand,  or  by  any  other 
mechanical  means,  would  carry  it  to  the  precise  height 
from  which  we  saw  it  fall. 

Now  the  lifting  of  the  weight  may  be  regarded  as 
so  much  mechanical  work  performed.  By  means  of  a 
ladder  placed  against  the  wall,  the  weight  might  be 
carried  up  to  a  height  of  16  feet;  or  it  might  be  drawn 
up  to  this  height  by  means  of  a  string  and  pulley,  or 
it  might  be  suddenly  jerked  up  to  a  height  of  16  feet. 
The  amount  of  work  done  in  all  these  cases,  as  far  as 
the  raising  of  the  weight  is  concerned,  would  be  abso- 
lutely the  same.  The  work  done  at  one  and  the  same 
*  A  discourse  delivered  in  the  Royal  Institution,  June  6, 1862. 


370  FBAGMENTS    OF    SCIENCE. 

place,  and  neglecting  the  small  change  of  gravity  with 
the  height,  depends  solely  upon  two  things;  on  the 
quantity  of  matter  lifted,  and  on  the  height  to  which 
it  is  lifted.  If  we  call  the  quantity  or  mass  of  matter 
m,  and  the  height  through  which  it  is  lifted  li,  then  the 
product  of  m  into  Ji,  or  m  h,  expresses,  or  is  propor- 
tional to,  the  amount  of  work  done. 

Supposing,  instead  of  imparting  a  velocity  of  32 
feet  a  second  we  impart  at  starting  twice  this  velocity. 
To  what  height  will  the  weight  rise?  You  might  be 
disposed  to  answer,  '  To  twice  the  height; '  but  this 
would  be  quite  incorrect.  Instead  of  twice  16,  or  32 
feet,  it  would  reach  a  height  of  four  times  16,  or  64 
feet.  So  also,  if  we  treble  the  starting  velocity,  the 
weight  would  reach  nine  times  the  height;  if  we  quad- 
ruple the  speed  at  starting,  we  attain  sixteen  times  the 
height.  Thus,  with  a  four-fold  velocity  of  128  feet  a 
second  at  starting,  the  weight  would  attain  an  elevation 
of  256  feet.  With  a  seven-fold  velocity  at  starting,  the 
weight  would  rise  to  49  times  the  height,  or  to  an  ele- 
vation of  784  feet. 

Now  the  work  done — or,  as  it  is  sometimes  called, 
the  mechanical  effect — other  things  being  constant,  is, 
as  before  explained,  proportional  to  the  height,  and 
as  a  double  velocity  gives  four  times  the  height,  a 
treble  velocity  nine  times  the  height,  and  so  on,  it  is 
perfectly  plain  that  the  mechanical  effect  increases  as 
the  square  of  the  velocity.  If  the  mass  of  the  body  be 
represented  by  the  letter  m,  and  its  velocity  by  v,  the 
mechanical  effect  would  be  proportional  to  or  repre- 
sented by  in  v2.  In  the  case  considered,  I  have  sup- 
posed the  weight  to  be  cast  upward,  being  opposed  in 
its  flight  by  the  resistance  of  gravity;  but  the  same 
holds  true  if  the  projectile  be  sent  into  water,  mud, 
earth,  timber,  or  other  resisting  material.  If,  for  ex- 


FORCE.  371 

ample,  we  double  the  velocity  of  a  cannon-ball,  we 
quadruple  its  mechanical  effect.  Hence  the  importance 
of  augmenting  the  velocity  of  a  projectile,  and  hence 
the  philosophy  of  Sir  William  Armstrong  in  using  a 
large  charge  of  powder  in  his  recent  striking  experi- 
ments. 

The  measure  then  of  mechanical  effect  is  the  mass 
of  the  body  multiplied  by  the  square  of  its  velocity. 

Now  in  firing  a  ball  against  a  target  the  projectile, 
after  collision,  is  often  found  hot.  Mr.  Fairbairn 
informs  me  that  in  the  experiments  at  Shoeburyness  it 
is  a  common  thing  to  see  a  flash,  even  in  broad  day- 
light, when  the  ball  strikes  the  target.  And  if  our  lead 
weight  be  examined  after  it  has  fallen  from  a  height 
it  is  also  found  heated.  Now  here  experiment  and 
reasoning  lead  us  to  the  remarkable  law  that,  like 
the  mechanical  effect,  the  amount  of  heat  generated  is 
proportional  to  the  product  of  the  mass  into  the  square 
of  the  velocity.  Double  your  mass,  other  things  being 
equal,  and  you  double  your  amount  of  heat;  double 
your  velocity,  other  things  remaining  equal,  and  you 
quadruple  your  amount  of  heat.  Here  then  we  have 
common  mechanical  motion  destroyed  and  heat  pro- 
duced. When  a  violin  bow  is  drawn  across  a  string, 
the  sound  produced  is  due  to  motion  imparted  to  the 
air,  and  to  produce  that  motion  muscular  force  has 
been  expended.  We  may  here  correctly  say,  that  the 
mechanical  force  of  the  arm  is  converted  into  music. 
In  a  similar  way  we  say  that  the  arrested  motion  of 
our  descending  weight,  or  of  the  cannon-ball,  is  con- 
verted into  heat.  The  mode  of  motion  changes,  but 
motion  still  continues;  the  motion  of  the  mass  is  con- 
verted into  a  motion  of  the  atoms  of  the  mass;  and 
these  small  motions,  communicated  to  the  nerves,  pro- 
duce the  sensation  we  call  heat. 


372  FKAGMENTS    OF    SCIENCE. 

We  know  the  amount  of  heat  which  a  given  amount 
of  mechanical  force  can  develope.  Our  lead  ball,  for 
example,  in  falling  to  the  earth  generated  a  quantity  of 
heat  sufficient  to  raise  its  own  temperature  three-fifths 
of  a  Fahrenheit  degree.  It  reached  the  earth  with  a 
velocity  of  32  feet  a  second,  and  forty  times  this  veloc- 
ity would  be  small  for  a  rifle  bullet;  multiplying  -fths 
by  the  square  of  40,  we  find  that  the  amount  of  heat 
developed  by  collision  with  the  target  would,  if  wholly 
concentrated  in  the  lead,  raise  its  temperature  960 
degrees.  This  would  be  more  than  sufficient  to  fuse 
the  lead.  In  reality,  however,  the  heat  developed  is 
divided  between  the  lead  and  the  body  against  which 
it  strikes;  nevertheless,  it  would  be  worth  while  to  pay 
attention  to  this  point,  and  to  ascertain  whether  rifle 
bullets  do  not,  under  some  circumstances,  show  signs 
of  fusion.* 

From  the  motion  of  sensible  masses,  by  gravity  and 
other  means,  we  now  pass  to  the  motion  of  atoms  to- 
wards each  other  by  chemical  affinity.  A  collodion 
balloon  filled  with  a  mixture  of  chlorine  and  hydrogen 
being  hung  in  the  focus  of  a  parabolic  mirror,  in  the 
focus  of  a  second  mirror  20  feet  distant  a  strong  elec- 
tric light  was  suddenly  generated;  the  instant  the 
concentrated  light  fell  upon  the  balloon,  the  gases 
within  it  exploded,  hydrochloric  acid  being  the  result. 
Here  the  atoms  virtually  fell  together,  the  amount  of 
heat  produced  showing  the  enormous  force  of  the  col- 
lision. The  burning  of  charcoal  in  oxygen  is  an  old 
experiment,  but  it  has  now  a  significance  beyond  what 
it  used  to  have;  we  now  regard  the  act  of  combination 
on  the  part  of  the  atoms  of  oxygen  and  coal  as  we  re- 

*  Eight  years  subsequently  this  surmise  was  proved  correct. 
In  the  Franco-German  War  signs  of  fusion  were  observed  in  the 
case  of  bullets  impinging  on  bones. 


FORCE.  373 

gard  the  clashing  of  a  falling  weight  against  the  earth. 
The  heat  produced  in  both  cases  is  referable  to  a  com- 
mon cause.  A  diamond,  which  burns  in  oxygen  as  a 
star  of  white  light,  glows  and  burns  in  consequence  of 
the  falling  of  the  atoms  of  oxygen  against  it.  And 
could  we  measure  the  velocity  of  the  atoms  when  they 
clash,  and  could  we  find  their  number  and  weights, 
multiplying  the  weight  of  each  atom  by  the  square  of 
its  velocity,  and  adding  all  together,  we  should  get  a 
number  representing  the  exact  amount  of  heat  de- 
veloped by  the  union  of  the  oxygen  and  carbon. 

Thus  far  we  have  regarded  the  heat  developed  by 
the  clashing  of  sensible  masses  and  of  atoms.  Work  is 
expended  in  giving  motion  to  these  atoms  or  masses, 
and  heat  is  developed.  But  we  reverse  this  process 
daily,  and  by  the  expenditure  of  heat  execute  work. 
We  can  raise  a  weight  by  heat;  and  in  this  agent  we 
possess  an  enormous  store  of  mechanical  power.  A 
pound  of  coal  produces  by  its  combination  with  oxygen 
an  amount  of  heat  which,  if  mechanically  applied, 
would  suffice  to  raise  a  weight  of  100  Ibs.  to  a  height  of 
20  miles  above  the  earth's  surface.  Conversely,  100 
Ibs.  falling  from  a  height  of  20  miles,  and  striking 
against  the  earth,  would  generate  an  amount  of  heat 
equal  to  that  developed  by  the  combustion  of  a  pound 
of  coal.  Wherever  work  is  done  by  heat,  heat  disap- 
pears. A  gun  which  fires  a  ball  is  less  heated  than  one 
which  fires  blank  cartridge.  The  quantity  of  heat  com- 
municated to  the  boiler  of  a  working  steam-engine  is 
greater  than  that  which  could  be  obtained  from  the 
re-condensation  of  the  steam,  after  it  had  done  its 
work;  and  the  amount  of  work  performed  is  the  exact 
equivalent  of  the  amount  of  heat  lost.  Mr.  Smyth  in- 
formed us  in  his  interesting  discourse,  that  we  dig 
annually  84  millions  of  tons  of  coal  from  our  pits. 


374  FRAGMENTS    OF    SCIENCE. 

The  amount  of  mechanical  force  represented  by  this 
quantity  of  coal  seems  perfectly  fabulous.  The  com- 
bustion of  a  single  pound  of  coal,  supposing  it  to  take 
place  in  a  minute,  would  be  equivalent  to  the  work  of 
300  horses;  and  if  we  suppose  108  millions  of  horses 
working  day  and  night  with  unimpaired  strength,  for 
a  year,  their  united  energies  would  enable  them  to  per- 
form an  amount  of  work  just  equivalent  to  that  which 
the  annual  produce  of  our  coal-fields  would  be  able 
to  accomplish. 

Comparing  with  ordinary  gravity  the  force  with 
which  oxygen  and  carbon  unite  together,  chemical 
affinity  seems  almost  infinite.  But  let  us  give  gravity 
fair  play  by  permitting  it  to  act  throughout  its  entire 
range.  Place  a  body  at  such  a  distance  from  the  earth 
that  the  attraction  of  our  planet  is  barely  sensible,  and 
let  it  fall  to  the  earth  from  this  distance.  It  would 
reach  the  earth  with  a  final  velocity  of  36,747  feet  a 
second;  and  on  collision  with  the  earth  the  body  would 
generate  about  twice  the  amount  of  heat  generated  by 
the  combustion  of  an  equal  weight  of  coal.  We  have 
stated  that  by  falling  through  a  space  of  16  feet  our 
lead  bullet  would  be  heated  three-fifths  of  a  degree; 
but  a  body  falling  from  an  infinite  distance  has  already 
used  up  1,299,999  parts  out  of  1,300,000  of  the  earth's 
pulling  power,  when  it  has  arrived  within  16  feet  of 
the  surface;  on  this  space  only  1>30Qi000ths  of  the  whole 
force  is  exerted. 

Let  us  now  turn  our  thoughts  for  a  moment  from 
the  earth  to  the  sun.  The  researches  of  Sir  John 
Herschel  and  M.  Pouillet  have  informed  us  of  the 
annual  expenditure  of  the  sun  as  regards  heat;  and  by 
an  easy  calculation  we  ascertain  the  precise  amount  of 
the  expenditure  which  falls  to  the  share  of  our  planet. 
Out  of  2,300  million  parts  of  light  and  heat  the  earth 


FORCE.  375 

receives  one.  The  whole  heat  emitted  by  the  sun  in  a 
minute  would  be  competent  to  boil  12,000  millions  of 
cubic  miles  of  ice-cold  water.  How  is  this  enormous 
loss  made  good — whence  is  the  sun's  heat  derived,  and 
by  what  means  is  it  maintained?  No  combustion — no 
chemical  affinity  with  which  we  are  acquainted,  would 
be  competent  to  produce  the  temperature  of  the  sun's 
surface.  Besides,  were  the  sun  a  burning  body  merely, 
its  light  and  heat  would  speedily  come  to  an  end. 
Supposing  it  to  be  a  solid  globe  of  coal,  its  combustion 
would  only  cover  4,600  years  of  expenditure.  In  this 
short  time  it  would  burn  itself  out.  What  agency 
then  can  produce  the  temperature  and  maintain  the 
outlay?  We  have  already  regarded  the  case  of  a  body 
falling  from  a  great  distance  towards  the  earth,  and 
found  that  the  heat  generated  by  its  collision  would  be 
twice  that  produced  by  the  combustion  of  an  equal 
weight  of  coal.  How  much  greater  must  be  the  heat 
developed  by  a  body  falling  against  the  sun!  The 
maximum  velocity  with  which  a  body  can  strike  the 
earth  is  about  7  miles  in  a  second;  the  maximum  ve- 
locity with  which  it  can  strike  the  sun  is  390  miles  in  a 
second.  And  as  the  heat  developed  by  the  collision  is 
proportional  to  the  square  of  the  velocity  destroyed, 
an  asteroid  falling  into  the  sun  with  the  above  veloc- 
ity would  generate  about  10,000  times  the  quantity  of 
heat  produced  by  the  combustion  of  an  asteroid  of  coal 
of  the  same  weight. 

Have  we  any  reason  to  believe  that  such  bodies 
exist  in  space,  and  that  they  may  be  raining  down  upon 
the  sun?  The  meteorites  flashing  through  the  air  are 
small  planetary  bodies,  drawn  by  the  earth's  attraction. 
They  enter  our  atmosphere  with  planetary  velocity, 
and  by  friction  against  the  air  they  are  raised  to  incan- 
descence and  caused  to  emit  light  and  heat.  At  cer- 


376  FEAGMENTS    OF    SCIENCE. 

tain  seasons  of  the  year  they  shower  down  upon  us  in 
great  numbers.  In  Boston  240,000  of  them  were  ob- 
served in  nine  hours.  There  is  no  reason  to  suppose 
that  the  planetary  system  is  limited  to  '  vast  masses  of 
enormous  weight; '  there  is,  on  the  contrary,  reason  to 
believe  that  space  is  stocked  with  smaller  masses,  which 
obey  the  same  laws  as  the  larger  ones.  That  lenticular 
envelope  which  surrounds  the  sun,  and  which  is  known 
to  astronomers  as  the  Zodiacal  light,  is  probably  a 
crowd  of  meteors;  and  moving  as  they  do  in  a  resisting 
medium,  they  must  continually  approach  the  sun. 
Falling  into  it,  they  would  produce  enormous  heat,  and 
this  would  constitute  a  source  from  which  the  annual 
loss  of  heat  might  be  made  good.  The  sun,  according 
to  this  hypothesis,  would  continually  grow  larger;  but 
how  much  larger?  Were  our  moon  to  fall  into  the 
sun,  it  would  develope  an  amount  of  heat  sufficient  to 
cover  one  or  two  years'  loss;  and  were  our  earth  to  fall 
into  the  sun  a  century's  loss  would  be  made  good. 
Still,  our  moon  and  our  earth,  if  distributed  over  the 
surface  of  the  sun,  would  utterly  vanish  from  percep- 
tion. Indeed,  the  quantity  of  matter  competent  to 
produce  the  required  effect  would,  during  the  range  of 
history,  cause  no  appreciable  augmentation  in  the  sun's 
magnitude.  The  augmentation  of  the  sun's  attractive 
force  would  be  more  sensible.  However  this  hypothesis 
may  fare  as  a  representant  of  what  is  going  on  in  na- 
ture, it  certainly  shows  how  a  sun  might  be  formed  and 
maintained  on  known  thermo-dynamic  principles. 

Our  earth  moves  in  its  orbit  with  a  velocity  of 
68,040  miles  an  hour.  Were  this  motion  stopped,  an 
amount  of  heat  would  be  developed  sufficient  to  raise 
the  temperature  of  a  globe  of  lead  of  the  same  size  as 
the  earth  384,000  degrees  of  the  centigrade  ther- 
mometer. It  has  been  prophesied  that  '  the  elements 


FORCE.  377 

shall  melt  with  fervent  heat.'  The  earth's  own  motion 
embraces  the  conditions  of  fulfilment;  stop  that  mo- 
tion, and  the  greater  part,  if  not  the  whole,  of  our 
planet  would  be  reduced  to  vapour.  If  the  earth  fell 
into  the  sun,  the  amount  of  heat  developed  by  the 
shock  would  be  equal  to  that  developed  by  the  combus- 
tion of  a  mass  of  solid  coal  6,435  times  the  earth  in  size. 
There  is  one  other  consideration  connected  with  the 
permanence  of  our  present  terrestrial  conditions,  which 
is  well  worthy  of  our  attention.  Standing  upon  one  of 
the  London  bridges,  we  observe  the  current  of  the 
Thames  reversed,  and  the  water  poured  upward  twice 
a-day.  The  water  thus  moved  rubs  against  the  river's 
bed,  and  heat  is  the  consequence  of  this  friction.  The 
heat  thus  generated  is  in  part  radiated  into  space  and 
lost,  as  far  as  the  earth  is  concerned.  What  supplies 
this  incessant  loss?  The  earth's  rotation.  Let  us  look 
a  little  more  closely  at  the  matter.  Imagine  the  moon 
fixed,  and  the  earth  turning  like  a  wheel  from  west  to 
east  in  its  diurnal  rotation.  Suppose  a  high  mountain 
on  the  earth's  surface  approaching  the  earth's  merid- 
ian; that  mountain  is,  as  it  were,  laid  hold  of  by  the 
moon;  it  forms  a  kind  of  handle  by  which  the  earth 
is  pulled  more  quickly  round.  But  when  the  meridian 
is  passed  the  pull  of  the  moon  on  the  mountain  would 
be  in  the  opposite  direction,  it  would  tend  to  diminish 
the  velocity  of  rotation  as  much  as  it  previously  aug- 
mented it;  thus  the  action  of  all  fixed  bodies  on  the 
earth's  surface  is  neutralised.  But  suppose  the  moun- 
tain to  lie  always  to  the  east  of  the  moon's  meridian, 
the  pull  then  would  be  always  exerted  against  the 
earth's  rotation,  the  velocity  of  which  would  be  dimin- 
ished in  a  degree  corresponding  to  the  strength  of  the 
pull.  The  tidal  wave  occupies  this  position — it  lies 
always  to  the  east  of  the  moon's  meridian.  The  waters 


378  FEAGMENTS    OF    SCIENCE. 

of  the  ocean  are  in  part  dragged  as  a  brake  along  the 
surface  of  the  earth;  and  as  a  brake  they  must  dimin- 
ish the  velocity  of  .the  earth's  rotation.*  Supposing 
then  that  we  turn  a  mill  by  the  action  of  the  tide,  and 
produce  heat  by  the  friction  of  the  millstones;  that 
heat  has  an  origin  totally 'different  from  the  heat  pro- 
duced by  another  mill  which  is  turned  by  a  mountain 
stream.  The  former  is  produced  at  the  expense  of  the 
earth's  rotation,  the  latter  at  the  expense  of  the  sun's 
radiation. 

The  sun,  by  the  act  of  vaporisation,  lifts  mechanic- 
ally all  the  moisture  of  our  air,  which  when  it  con- 
denses falls  in  the  form  of  rain,  and  when  it  freezes 
falls  as  snow.  In  this  solid  form  it  is  piled  upon  the 
Alpine  heights,  and  furnishes  materials  for  glaciers. 
But  the  sun  again  interposes,  liberates  the  solidified 
liquid,  and  permits  it  to  roll  by  gravity  to  the  sea.  The 
mechanical  force  of  every  river  in  the  world  as  it  rolls 
towards  the  ocean,  is  drawn  from  the  heat  of  the  sun. 
No  streamlet  glides  to  a  lower  level  without  having 
been  first  lifted  to  the  elevation  from  which  it  springs 
by  the  power  of  the  sun.  The  energy  of  winds  is  also 
due  entirely  to  the  same  power. 

But  there  is  still  another  work  which  the  sun  per- 
forms, and  its  connection  with  which  is  not  so  obvious. 
Trees  and  vegetables  grow  upon  the  earth,  and  when 
burned  they  give  rise  to  heat,  and  hence  to  mechanical 
energy.  Whence  is  this  power  derived?  You  see  this 
oxide  of  iron,  produced  by  the  falling  together  of  the 
atoms  of  iron  and  oxygen;  you  cannot  see  this  trans- 
parent carbonic  acid  gas,  formed  by  the  falling  to- 
gether of  carbon  and  oxygen.  The  atoms  thus  in  close 
union  resemble  our  lead  weight  while  resting  on  the 

*  Kant  surmised  an  action  of  this  kind. 


FORCE.  379 

earth;  but  we  can  wind  up  the  weight  and  prepare  it 
for  another  fall,  and  so  these  atoms  can  be  wound  up 
and  thus  enabled  to  repeat  the  process  of  combination. 
In  the  building  of  plants  carbonic  acid  is  the  material 
from  which  the  carbon  of  the  plant  is  derived;  and 
the  solar  beam  is  the  agent  which  tears  the  atoms 
asunder,  setting  the  oxygen  free,  and  allowing  the 
carbon  to  aggregate  in  woody  fibre.  Let  the  solar  rays 
fall  upon  a  surface  of  sand;  the  sand  is  heated,  and 
finally  radiates  away  as  much  heat  as  it  receives;  let 
the  same  beams  fall  upon  a  forest,  the  quantity  of  heat 
given  back  is  less  than  the  forest  receives;  for  the 
energy  of  a  portion  of  the  sunbeams  is  invested  in 
building  the  trees.  Without  the  sun  the  reduction  of 
the  carbonic  acid  cannot  be  effected,  and  an  amount  of 
sunlight  is  consumed  exactly  equivalent  to  the  molec- 
ular work  done.  Thus  trees  are  formed;  thus  the 
cotton  on  which  Mr.  Bazley  discoursed  last  Friday  is 
produced.  I  ignite  this  cotton,  and  it  flames;  the  oxy- 
gen again  unites  with  the  carbon;  but  an  amount  of 
heat  equal  to  that  produced  by  its  combustion  was 
sacrificed  by  the  sun  to  form  that  bit  of  cotton. 

We  cannot,  however,  stop  at  vegetable  life,  for  it 
is  the  source,  mediate  or  immediate,  of  all  animal  life. 
The  sun  severs  the  carbon  from  its  oxygen  and  builds 
the  vegetable;  the  animal  consumes  the  vegetable 
thus  formed,  a  reunion  of  the  severed  elements  takes 
place,  producing  animal  heat.  The  process  of  building 
a  vegetable  is  one  of  winding  up;  the  process  of  build- 
ing an  animal  is  one  of  running  down.  The  warmth 
of  our  bodies,  and  every  mechanical  energy  which  we 
exert,  trace  their  lineage  directly  to  the  sun.  The 
fight  of  a  pair  of  pugilists,  the  motion  of  an  army,  or 
the  lifting  of  his  own  body  by  an  Alpine  climber  up 
a  mountain  slope,  are  all  cases  of  mechanical  energy 


380  FKAGMENTS    OF    SCIENCE. 

drawn  from  the  sun.  A  man  weighing  150  pounds  has 
64  pounds  of  muscle;  but  these,  when  dried,  reduce 
themselves  to  15  pounds.  Doing  an  ordinary  day's 
work,  for  eighty  days,  this  mass  of  muscle  would  be 
wholly  oxidised.  Special  organs  which  do  more  work 
would  be  more  quickly  consumed:  the  heart,  for  ex- 
ample, if  entirely  unsustained,  would  be  oxidised  in 
about  a  week.  Take  the  amount  of  heat  due  to  the 
direct  oxidation  of  a  given  weight  of  food;  less  heat 
is  developed  by  the  oxidation  of  the  same  amount  of 
food  in  the  working  animal  frame,  and  the  missing 
quantity  is  the  equivalent  of  the  mechanical  work  ac- 
complished by  the  muscles. 

I  might  extend  these  considerations;  the  work, 
indeed,  is  done  to  my  hand — but  I  am  warned  that 
you  have  been  already  kept  too  long.  To  whom  then 
are  we  indebted  for  the  most  striking  generalisations 
of  this  evening's  discourse?  They  are  the  work  of 
a  man  of  whom  you  have  scarcely  ever  heard — the 
published  labours  of  a  German  doctor,  named  Mayer. 
Without  external  stimulus,  and  pursuing  his  profes- 
sion as  town  physician  in  Heilbronn,  this  man  was  the 
first  to  raise  the  conception  of  the  interaction  of  heat 
and  other  natural  forces  to  clearness  in  his  own  mind. 
And  yet  he  is  scarcely  ever  heard  of,  and  even  to  scien- 
tific men  his  merits  are  but  partially  known.  Led  by 
his  own  beautiful  researches,  and  quite  independent 
of  Mayer,  Mr.  Joule  published  in  1843  his  first  paper 
on  the  '  Mechanical  Value  of  Heat; '  but  in  1842  Mayer 
had  actually  calculated  the  mechanical  equivalent  of 
heat  from  data  which  only  a  man  of  the  rarest  pene- 
tration could  turn  to  account.  In  1845  he  published 
his  memoir  on  '  Organic  Motion,'  and  applied  the 
mechanical  theory  of  heat  in  the  most  fearless  and  pre- 
cise manner  to  vital  processes.  He  also  embraced  the 


FORCE.  381 

other  natural  agents  in  his  chain  of  conservation.  In 
1858  Mr.  Waterston  proposed,  independently,  the  me- 
teoric theory  of  the  sun's  heat,  and  in  1854  Professor 
William  Thomson  applied  his  admirable  mathematical 
powers  to  the  development  of  the  theory;  but  six  years 
previously  the  subject  had  been  handled  in  a  masterly 
manner  by  Mayer,  and  all  that  I  have  said  about  it  has 
been  derived  from  him.  When  we  consider  the  cir- 
cumstances of  Mayer's  life,  and  the  period  at  which  he 
wrote,  we  cannot  fail  to  be  struck  with  astonishment  at 
what  he  has  accomplished.  Here  was  a  man  of  genius 
working  in  silence,  animated  solely  by  a  love  of  his 
subject,  and  arriving  at  the  most  important  results  in 
advance  of  those  whose  lives  were  entirely  devoted  to 
Natural  Philosophy.  It  was  the  accident  of  bleeding  a 
feverish  patient  at  Java  in  1840  that  led  Mayer  to 
speculate  on  these  subjects.  He  noticed  that  the  venous 
blood  in  the  tropics  was  of  a  brighter  red  than  in  colder 
latitudes,  and  his  reasoning  on  this  fact  led  him  into 
the  laboratory  of  natural  forces,  where  he  has  worked 
with  such  signal  ability  and  success.  Well,  you  will 
desire  to  know  what  has  become  of  this  man.  His 
mind,  it  is  alleged,  gave  way;  it  is  said  he  became  in- 
sane, and  he  was  certainly  sent  to  a  lunatic  asylum. 
In  a  biographical  dictionary  of  his  country  it  is  stated 
that  he  died  there,  but  this  is  incorrect.  Hfc  recovered; 
and,  I  believe,  is  at  this  moment  a  cultivator  of  vine- 
yards in  Heilbronn. 


382  FRAGMENTS    OF    SCIENCE. 

June  20,  1862. 

While  preparing  for  publication  my  last  course  of 
lectures  on  Heat,  I  wished  to  make  myself  acquainted 
with  all  that  Dr.  Mayer  had  done  in  connection  with 
this  subject.  I  accordingly  wrote  to  two  gentlemen 
who  above  all  others  seemed  likely  to  give  me  the  in- 
formation which  I  needed.*  Both  of  them  are  Ger- 
mans, and  both  particularly  distinguished  in  connec- 
tion with  the  Dynamical  Theory  of  Heat.  Each  of 
them  kindly  furnished  me  with  the  list  of  Mayer's  pub- 
lications, and  one  of  them  [Clausius]  was  so  friendly 
as  to  order  them  from  a  bookseller,  and  to  send  them 
to  me.  This  friend,  in  his  reply  to  my  first  letter  re- 
garding Mayer,  stated  his  belief  that  I  should  not  find 
anything  very  important  in  Mayer's  writings;  but  be- 
fore forwarding  the  memoirs  to  me  he  read  them  him- 
self. His  letter  accompanying  them  contains  the  fol- 
lowing words: — 'I  must  here  retract  the  statement  in 
my  last  letter,  that  you  would  not  find  much  matter 
of  importance  in  Mayer's  writings:  I  am  astonished  at 
the  multitude  of  beautiful  and  correct  thoughts  which 
they  contain; '  and  he  goes  on  to  point  out  various  im- 
portant subjects,  in  the  treatment  of  which  Mayer  had 
anticipated  other  eminent  writers.  My  other  friend, 
in  whose  own  publications  the  name  of  Mayer  repeat- 
edly occurs,  and  whose  papers  containing  these  refer- 
ences were  translated  some  years  ago  by  myself,  was, 
on  the  10th  of  last  month,  unacquainted  with  the 
thoughtful  and  beautiful  essay  of  Mayer's,  entitled 
'Beitrage  zur  Dynamik  des  Himmels/  and  in  1854, 
when  Professor  William  Thomson  developed  in  so 
striking  a  manner  the  meteoric  theory  of  the  sun's 
heat,  he  was  certainly  not  aware  of  the  existence  of 

*  Helmholtz  and  Clausius. 


FORCE.  383 

that  essay,  though  from  a  recent  article  in  '  Macmillan's 
Magazine '  I  infer  that  he  is  now  aware  of  it.  Mayer's 
physiological  writings  have  been  referred  to  by  physi- 
ologists— by  Dr.  Carpenter,  for  example — in  terms  of 
honouring  recognition.  We  have  hitherto,  indeed,  ob- 
tained fragmentary  glimpses  of  the  man,  partly  from 
physicists  and  partly  from  physiologists;  but  his  total 
merit  has  never  yet  been  recognised  as  it  assuredly 
would  have  been  had  he  chosen  a  happier  mode  of 
publication.  I  do  not  think  a  greater  disservice  could 
be  done  to  a  man  of  science,  than  to  overstate  his 
claims:  such  overstatement  is  sure  to  recoil  to  the  dis- 
advantage of  him  in  whose  interest  it  is  made.  But 
when  Mayer's  opportunities,  achievements,  and  fate 
are  taken  into  account,  I  do  not  think  that  I  shall  be 
deeply  blamed  for  attempting  to  place  him  in  that 
honourable  position,  which  I  believe  to  be  his  due. 

Here,  however,  are  the  titles  of  Mayer's  papers,  the 
perusal  of  which  will  correct  any  error  of  judgment 
into  which  I  may  have  fallen  regarding  their  author. 
'  Bemerkungen  iiber  die  Krafte  der  unbelebten  Natur,' 
Liebig's  '  Annalen,'  1842,  Vol.  42,  p.  231; '  Die  Organ- 
ische  Bewegung  in  ihrem  Zusammenhange  mit  dem 
Stoffwechsel,'  Heilbronn,  1845; '  Beitrage  zur  Dynamik 
des  Himmels,'  Heilbronn,  1848;  'Bemerkungen  iiber 
das  Mechanische  Equivalent  der  Wanne/  Heilbronn, 
1851. 


IN  MEMORIAM. — Dr.  Julius  Robert  Mayer  died  at 
Heilbronn  on  March  20,  1878,  aged  63  years.  It  gives 
me  pleasure  to  reflect  that  the  great  position  which  he 
will  for  ever  occupy  in  the  annals  of  science  was  first 
virtually  assigned  to  him  in  the  foregoing  discourse. 
He  was  subsequently  chosen  by  acclamation  a  member 


384  FKAGMENTS    OF    SCIENCE. 

of  the  French  Academy  of  Sciences;  and  he  received 
from  the  Eoyal  Society  the  Copley  medal — its  highest 
reward.* 


November  1878. 

At  the  meeting  of  the  British  Association  at  Glas- 
gow in  1876 — that  is  to  say,  more  than  fourteen  years 
after  its  delivery  and  publication — the  foregoing  lec- 
ture was  made  the  cloak  for  an  unseemly  personal  at- 
tack by  Professor  Tait.  The  anger  which  found  this 
uncourteous  vent  dates  from  1863, f  when  it  fell  to  my 
lot  to  maintain,  in  opposition  to  him  and  a  more  emi- 
nent colleague,  the  position  which  in  1862  I  had  as- 
signed to  Dr.  Mayer.  In  those  days  Professor  Tait  de- 
nied to  Mayer  all  originality,  and  he  has  since,  I  regret 
to  say,  never  missed  an  opportunity,  however  small,  of 
carping  at  Mayer's  claims.  The  action  of  the  Academy 
of  Sciences  and  of  the  Eoyal  Society  summarily  dis- 
poses of  this  detraction,  to  which  its  object,  during  his 
lifetime,  never  vouchsafed  either  remonstrance  or  reply. 

Some  time  ago  Professor  Tait  published  a  volume 
of  lectures  entitled  '  Eecent  Advances  in  Physical  Sci- 
ence,' which  I  have  reason  to  know  has  evoked  an 
amount  of  censure  far  beyond  that  hitherto  publicly 
expressed.  Many  of  the  best  heads  on  the  continent 
of  Europe  agree  in  their  rejection  and  condemnation 
of  the  historic  portions  of  this  book.  In  March  last 
it  was  subjected  to  a  brief  but  pungent  critique  by  Du 
Bois-Eeymond,  the  celebrated  Perpetual  Secretary  of 
the  Academy  of  Sciences  in  Berlin.  Du  Bois-Eeymond's 
address  was  on  '  National  Feeling,'  and  his  critique  is 

*  See  'The  Copley  Medalist  for  1871,'  p.  479. 
f  See  '  Philosophical  Magazine '  for  this  and  the  succeeding 
years. 


FORCE.  385 

thus  wound  up: — 'The  author  of  the  "Lectures"  is 
not,  perhaps,  sufficiently  well  acquainted  with  the  his- 
tory on  which  he  professes  to  throw  light,  and  on  the 
later  phases  of  which  he  passes  so  unreserved  (schroff) 
a  judgment.  He  thus  exposes  himself  to  the  suspicion 
— which,  unhappily,  is  not  weakened  by  his  other  writ- 
ings— that  the  fiery  Celtic  blood  of  his  country  occa- 
sionally runs  away  with  him,  converting  him  for  the 
time  into  a  scientific  Chauvin.  Scientific  Chauvinism/ 
adds  the  learned  secretary,  'from  which  German  in- 
vestigators have  hitherto  kept  free,  is  more  reprehensi- 
ble (gehassig)  than  political  Chauvinism,  inasmuch  as 
self-control  (sittliche  Haltung)  is  more  to  be  expected 
from  men  of  science,  than  from  the  politically  excited 
mass.'  * 

In  the  case  before  this  '  expectation '  would,  I  fear, 
be  doomed  to  disappointment.  But  Du  Bois-Reymond 
and  his  countrymen  must  not  accept  the  writings  of 
Professor  Tait  as  representative  of  the  thought  of  Eng- 
land. Surely  no  nation  in  the  world  has  more  effec- 
tually shaken  itself  free  from  scientific  Chauvinism. 
From  the  day  that  Davy,  on  presenting  the  Copley 
medal  to  Arago,  scornfully  brushed  aside  that  spurious 
patriotism  which  would  run  national  boundaries 
through  the  free  domain  of  science,  chivalry  towards 
foreigners  has  been  a  guiding  principle  with  the  Royal 
Society. 

On  the  more  private  amenities  indulged  in  by  Pro- 
fessor Tait,  I  do  not  consider  it  necessary  to  say  a 
word. 

*  Festrede,  delivered  before  .the  Academy  of  Sciences  of  Ber- 
lin, in  celebration  of  the  birthday  of  the  Emperor  and  King, 
March  28, 1878. 


XVII. 

CONTRIBUTIONS  TO  MOLECULAR  PHYSICS* 

HAVING  on  previous  occasions  dwelt  upon  the 
enormous  differences  which  exist  among  gase- 
ous bodies  both  as  regards  their  power  of  absorbing 
and  emitting  radiant  heat,  I  have  now  to  consider  the 
effect  of  a  change  of  aggregation.  When  a  gas  is  con- 
densed to  a  liquid,  or  a  liquid  congealed  to  a  solid,  the 
molecules  coalesce,  and  grapple  with  each  other  by 
forces  which  are  insensible  as  long  as  the  gaseous  state 
is  maintained.  But,  even  in  the  solid  and  liquid  con- 
ditions, the  luminiferous  ether  still  surrounds  the  mole- 
cules: hence,  if  the  acts  of  radiation  and  absorption 
depend  on  them  individually,  regardless  of  their  state 
of  aggregation,  the  change  from  the  gaseous  to  the 
liquid  state  ought  not  materially  to  affect  the  radiant 
and  absorbent  power.  If,  on  the  contrary,  the  mutual 
entanglement  of  the  molecules  by  the  force  of  cohesion 
be  of  paramount  influence,  then  we  may  expect  that 
liquids  will  exhibit  a  deportment  towards  radiant  heat 
altogether  different  from  that  of  the  vapours  from 
which  they  are  derived. 

The  first  part  of  an  enquiry  conducted  in  1863-64 
was  devoted  to  an  exhaustive  examination  of  this  ques- 
tion. Twelve  different  liquids  were  employed,  and  five 

*  A  discourse  delivered  at  the  Royal  Institution,  March  18, 
1864 — supplementing,  though  of  prior  date,  the  Rede  Lecture  on 
Radiation. 


CONTRIBUTIONS    TO    MOLECULAR    PHYSICS.    387 

different  layers  of  each,  varying  in  thickness  from  0.02 
of  an  inch  to  0.27  of  an  inch.  The  liquids  were  en- 
closed, not  in  glass  vessels,  which  would  have  material- 
ly modified  the  incident  heat,  but  between  plates  of 
transparent  rock-salt,  which  only  slightly  affected  the 
radiation.  The  source  of  heat  throughout  these  com- 
parative experiments  consisted  of  a  platinum  wire, 
raised  to  incandescence  by  an  electric  current  of  un- 
varying strength.  The  quantities  of  radiant  heat  ab- 
sorbed and  transmitted  by  each  of  the  liquids  at  the 
respective  thicknesses  were  first  determined.  The 
vapours  of  these  liquids  were  subsequently  examined, 
the  quantities  of  vapour  employed  being  rendered  pro- 
portional to  the  quantities  of  liquid  previously  trav- 
ersed by  the  radiant  heat.  The  result  was  that,  for 
heat  from  the  same  source,  the  order  of  absorption  of 
liquids  and  of  their  vapours  proved  absolutely  the 
same.  There  is  no  known  exception  to  this  law;  so 
that,  to  determine  the  position  of  a  vapour  as  an  ab- 
sorber or  a  radiator,  it  is  only  necessary  to  determine 
the  position  of  its  liquid. 

This  result  proves  that  the  state  of  aggregation,  as 
far  at  all  events  as  the  liquid  stage  is  concerned,  is  of 
altogether  subordinate  moment — a  conclusion  which 
will  probably  prove  to  be  of  cardinal  importance  in 
molecular  physics.  On  one  important  and  contested 
point  it  has  a  special  bearing.  If  the  position  of  a 
liquid  as  an  absorber  and  radiator  determine  that  of  its 
vapour,  the  position  of  water  fixes  that  of  aqueous 
vapour.  Water  has  been  compared  with  other  liquids 
in  a  multitude  of  experiments,  and  it  has  been  found, 
both  as  a  radiant  and  as  an  absorbent,  to  transcend 
them  all.  Thus,  for  example,  a  layer  of  bisulphide  of 
carbon  0.02  of  an  inch  in  thickness  absorbs  6  per  cent., 
and  allows  94  per  cent,  of  the  radiation  from  the  red- 


388  FRAGMENTS    OF    SCIENCE. 

hot  platinum  spiral  to  pass  through  it;  benzol  absorbs 
43  and  transmits  57  per  cent,  of  the  same  radiation; 
alcohol  absorbs  67  and  transmits  33  per  cent.,  and  alco- 
hol, as  an  absorber  of  radiant  heat,  stands  at  the  head 
of  all  liquids  except  one.  The  exception  is  water.  A 
layer  of  this  substance,  of  the  thickness  above  given, 
absorbs  81  per  cent.,  and  permits  only  19  per  cent,  of 
the  radiation  to  pass  through  it.  Had  no  single  experi- 
ment ever  been  made  upon  the  vapour  of  water,  its 
vigorous  action  upon  radiant  heat  might  be  inferred 
from  the  deportment  of  the  liquid. 

The  relation  of  absorption  and  radiation  to  the 
chemical  constitution  of  the  radiating  and  absorbing 
substances  was  next  briefly  considered.  For  the  first 
six  substances  in  the  list  of  liquids  examined,  the  radi- 
ant and  absorbent  powers  augment  as  the  number  of 
atoms  in  the  compound  molecule  augments.  Thus, 
bisulphide  of  carbon  has  3  atoms,  chloroform  5,  iodide 
of  ethyl  8,  benzol  12,  and  amylene  15  atoms  in  their 
respective  molecules.  The  order  of  their  power  as  radi- 
ants and  absorbents  is  that  here  indicated,  bisulphide 
of  carbon  being  the  feeblest,  and  amylene  the  strongest 
of  the  six.  Alcohol,  however,  excels  benzol  as  an  ab- 
sorber, though  it  has  but  9  atoms  in  its  molecule;  but, 
on  the  other  hand,  its  molecule  is  rendered  more  com- 
plex by  the  introduction  of  a  new  element.  Benzol 
contains  carbon  and  hydrogen,  while  alcohol  contains 
carbon,  hydrogen  and  oxygen.  Thus,  not  only  does 
atomic  multitude  come  into  play  in  absorption  and 
radiation — atomic  complexity  must  also  be  taken  into 
account.  I  would  recommend  to  the  particular  atten- 
tion of  chemists  the  molecule  of  water;  the  deportment 
of  this  substance  towards  radiant  heat  being  perfectly 
anomalous,  if  the  chemical  formula  at  present  ascribed 
to  it  be  correct. 


CONTRIBUTIONS    TO    MOLECULAR    PHYSICS.    389 

Sir  William  Herschel  made  the  important  discov- 
ery that,  beyond  the  limit?  of  the  red  end  of  the  solar 
spectrum,  rays  of  high  heating  power  exist  which  are 
incompetent  to  excite  vision.  The  discovery  is  capa- 
ble of  extension.  Dissolving  iodine  in  the  bisulphide 
of  carbon,  a  solution  is  obtained  which  entirely  inter- 
cepts the  light  of  the  most  brilliant  flames,  while  to  the 
ultra-red  rays  of  such  flames  the  same  iodine  is  found 
to  be  perfectly  diathermic.  The  transparent  bisul- 
phide, which  is  highly  pervious  to  invisible  heat,  ex- 
ercises on  it  the  same  absorption  as  the  perfectly 
opaque  solution.  A  hollow  prism  filled  with  the  opaque 
liquid  being  placed  in  the  path  of  the  beam  from  an 
electric  lamp,  the  light-spectrum  is  completely  inter- 
cepted, but  the  heat-spectrum  may  be  received  upon  a 
screen  and  there  examined.  Falling  upon  a  thermo- 
electric pile,  its  invisible  presence  is  shown  by  the 
prompt  deflection  of  even  a  coarse  galvanometer. 

What,  then,  is  the  physical  meaning  of  opacity  and 
transparency  as  regards  light  and  radiant  heat?  The 
visible  rays  of  the  spectrum  differ  from  the  invisible 
ones  simply  in  period.  The  sensation  of  light  is  ex- 
cited by  waves  of  ether  shorter  and  more  quickly  re- 
current than  the  non-visual  waves  which  fall  beyond 
the  extreme  red.  But  why  should  iodine  stop  the 
former  and  allow  the  latter  to  pass?  The  answer  to 
this  question  no  doubt  is,  that  the  intercepted  waves 
are  those  whose  periods  of  recurrence  coincide  with  the 
periods  of  oscillation  possible  to  the  atoms  of  the  dis- 
solved iodine.  The  elastic  forces  which  keep  these 
atoms  apart  compel  them  to  vibrate  in  definite  periods, 
and,  when  these  periods  synchronise  with  those  of  the 
ethereal  waves,  the  latter  are  absorbed.  Briefly  de- 
fined, then,  transparency  in  liquids,  as  well  as  in  gases, 
is  synonymous  with  discord,  while  opacity  is  synony- 


390  FRAGMENTS    OF    SCIENCE. 

mous  with  accord,  between  the  periods  of  the  waves  of 
ether  and  those  of  the  molecules  on  which  they  im- 
pinge. 

According  to  this  view  transparent  and  colourless 
substances  owe  their  transparency  to  the  dissonance 
existing  between  the  oscillating  periods  of  their  atoms 
and  those  of  the  waves  of  the  whole  visible  spectrum. 
From  the  prevalence  of  transparency  in  compound 
bodies,  the  general  discord  of  the  vibrating  periods  of 
their  atoms  with  the  light-giving  waves  of  the  spec- 
trum,, may  be  inferred;  while  their  synchronism  with 
the  ultra-red  periods  is  to  be  inferred  from  their  opac- 
ity to  the  ultra-red  rays.  Water  illustrates  this  in  a 
most  striking  manner.  It  is  highly  transparent  to  the 
luminous  rays,  which  proves  that  its  atoms  do  not 
readily  oscillate  in  the  periods  which  excite  vision.  It 
is  highly  opaque  to  the  ultra-red  undulations,  which 
proves  the  synchronism  of  its  vibrating  periods  with 
those  of  the  longer  waves. 

If,  then,  to  the  radiation  from  any  source  water 
shows  itself  eminently  or  perfectly  opaque,  we  may 
infer  that  the  atoms  whence  the  radiation  emanates 
oscillate  in  ultra-red  periods.  Let  us  apply  this  test 
to  the  radiation  from  a  flame  of  hydrogen.  This  flame 
consists  mainly  of  incandescent  aqueous  vapour,  the 
temperature  of  which,  as  calculated  by  Bunsen,  is 
3259°  C.,  so  that,  if  the  penetrative  power  of  radiant 
heat,  as  generally  supposed,  augment  with  the  tem- 
perature of  its  source,  we  may  expect  the  radiation 
from  this  flame  to  be  copiously  transmitted  by  water. 
While,  however,  a  layer  of  the  bisulphide  of  carbon 
0.07  of  an  inch  in  thickness  transmits  72  per  cent,  of 
the  incident  radiation,  and  while  every  other  liquid 
examined  transmits  more  or  less  of  the  heat,  a  layer 
of  water  of  the  above  thickness  is  entirely  opaque  to 


CONTRIBUTIONS    TO    MOLECULAR    PHYSICS.    391 

the  radiation  from  the  hydrogen  flame.  Thus  we  es- 
tablish accord  between  the  periods  of  the  atoms  of  cold 
water  and  those  of  aqueous  vapour  at  a  temperature 
of  3259°  C.  But  the  periods  of  water  have  already 
been  proved  to  be  ultra-red — hence  those  of  the  hydro- 
gen flame  must  be  sensibly  ultra-red  also.  The  ab- 
sorption by  dry  air  of  the  heat  emitted  by  a  platinum 
spiral  raised  to  incandescence  by  electricity  is  insensi- 
ble, while  that  by  the  ordinary  undried  air  is  6  per 
cent.  Substituting  for  the  platinum  spiral  a  hydro- 
gen flame,  the  absorption  by  dry  air  still  remains  in- 
sensible, while  that  of  the  undried  air  rises  to  20  per 
cent,  of  the  entire  radiation.  The  temperature  of  the 
hydrogen  flame  is,  as  stated,  3259°  C.;  that  of  the  aque- 
ous vapour  of  the  air  20°  C.  Suppose,  then,  the  tem- 
perature of  aqueous  vapour  to  rise  from  20°  C.  to 
3259°  C.,  we  must  conclude  that  the  augmentation  of 
temperature  is  applied  to  an  increase  of  amplitude  or 
width  of  swing,  and  not  to  the  introduction  of  quicker 
periods  into  the  radiation. 

The  part  played  by  aqueous  vapour  in  the  economy 
of  nature  is  far  more  wonderful  than  has  been  hitherto 
supposed.  To  nourish  the  vegetation  of  the  earth 
the  actinic  and  luminous  rays  of  the  sun  must  pene- 
trate our  atmosphere;  and  to  such  rays  aqueous  vapour 
is  eminently  transparent.  The  violet  and  the  ultra- 
violet rays  pass  through  it  with  freedom.  To  protect 
vegetation  from  destructive  chills  the  terrestrial  rays 
must  be  checked  in  their  transit  towards  stellar  space; 
and  this  is  accomplished  by  the  aqueous  vapour  dif- 
fused through  the  air.  This  substance  is  the  great 
moderator  of  the  earth's  temperature,  bringing  its  ex- 
tremes into  proximity,  and  obviating  contrasts  between 
day  and  night  which  would  render  life  insupportable. 
But  we  can  advance  beyond  this  general  statement, 


392  FKAGMENTS    OF    SCIENCE. 

now  that  we  know  the  radiation  from  aqueous  vapour 
is  intercepted,  in  a  special  degree,  by  water,  and,  re- 
ciprocally, the  radiation  from  water  by  aqueous  va- 
pour; for  it  follows  from  this  that  the  very  act  of  noc- 
turnal refrigeration  which  produces  the  condensation 
of  aqueous  vapour  at  the  surface  of  the  earth — giving, 
as  it  were,  a  varnish  of  water  to  that  surface — imparts 
to  terrestrial  radiation  that  particular  character  which 
disqualifies  it  from  passing  through  the  earth's  atmos- 
phere and  losing  itself  in  space. 

And  here  we  come  to  a  question  in  molecular  phys- 
ics which  at  the  present  moment  occupies  attention. 
By  allowing  the  violet  and  ultra-violet  rays  of  the  spec- 
trum to  fall  upon  sulphate  of  quinine  and  other  sub- 
stances Professor  Stokes  has  changed  the  periods  of 
those  rays.  Attempts  have  been  made  to  produce  a 
similar  result  at  the  other  end  of  the  spectrum — to 
convert  the  ultra-red  periods  into  periods  competent 
to  excite  vision — but  hitherto  without  success.  Such 
a  change  of  period,  I  agree  with  Dr.  Miller  in  believ- 
ing, occurs  when  the  lime-light  is  produced  by  an  oxy- 
hydrogen  flame.  In  this  common  experiment  there  is 
an  actual  breaking  up  of  long  periods  into  short  ones — 
a  true  rendering  of  unvisual  periods  visual.  The 
change  of  refrangibility  here  effected  differs  from  that 
of  Professor  Stokes;  firstly,  by  its  being  in  the  oppo- 
site direction — that  is,  from  a  lower  refrangibility  to  a 
higher;  and  secondly,  in  the  circumstance  that  the 
lime  is  heated  by  the  collision  of  the  molecules  of  aque- 
ous vapour,  before  their  heat  has  assumed  the  radiant 
form.  But  it  cannot  be  doubted  that  the  same  effect 
would  be  produced  by  radiant  heat  of  the  same  periods, 
provided  the  motion  of  the  ether  could  be  rendered 
.sufficiently  intense.*  The  effect  in  principle  is  the 

*  This  was  soon  afterwards  accomplished.    See  pp.  48,  49. 


CONTRIBUTIONS    TO    MOLECULAR    PHYSICS.     393 

same,  whether  we  consider  the  lime  to  be  struck  by 
a  particle  of  aqueous  vapour  oscillating  at  a  certain  rate, 
or  by  a  particle  of  ether  oscillating  at  the  same  rate. 

By  plunging  a  platinum  wire  into  a  hydrogen  flame 
we  cause  it  to  glow,  and  thus  introduce  shorter  periods 
into  the  radiation.  These,  as  already  stated,  are  in 
discord  with  the  atomic  vibrations  of  water;  hence  we 
may  infer  that  the  transmission  through  water  will  be 
rendered  more  copious  by  the  introduction  of  the  wire 
into  the  flame.  Experiment  proves  this  conclusion  to 
be  true.  Water,  from  being  opaque,  opens  a  passage  to 
6  per  cent,  of  the  radiation  from  the  spiral.  A  thin 
plate  of  colourless  glass,  moreover,  transmits  58  per 
cent,  of  the  radiation  from  the  hydrogen  flame;  but 
when  the  flame  and  spiral  are  employed,  78  per  cent, 
of  the  heat  is  transmitted. 

For  an  alcohol  flame  Knoblauch  and  Melloni  found 
glass  to  be  less  transparent  than  for  the  same  flame 
with  a  platinum  spiral  immersed  in  it;  but  Melloni 
afterwards  showed  that  the  result  was  not  general — 
that  black  glass  and  black  mica  were  decidedly  more 
diathermic  to  the  radiation  from  the  pure  alcohol 
flame.  Melloni  did  not  explain  this,  but  the  reason  is 
now  obvious.  The  mica  and  glass  owe  their  blackness 
to  the  carbon  diffused  through  them.  This  carbon,  as 
first  proved  by  Melloni,  is  in  some  measure  transparent 
to  the  ultra-red  rays,  and  I  have  myself  succeeded  in 
transmitting  between  40  and  50  per  cent,  of  the  radia- 
tion from  a  hydrogen  flame  through  a  layer  of  carbon 
which  intercepted  the  light  of  an  intensely  brilliant 
flame.  The  products  of  combustion  of  alcohol  are  car- 
bonic acid  and  aqueous  vapour,  the  heat  of  which  is 
almost  wholly  ultra-red.  For  this  radiation,  then,  the 
carbon  is  in  a  considerable  degree  transparent,  while 
for  the  radiation  from  the  platinum  spiral,  it  is  in  a 


394  FRAGMENTS    OF    SCIENCE. 

great  measure  opaque.  The  platinum  wire,  therefore, 
which  augmented  the  radiation  through  the  pure  glass, 
augmented  the  absorption  of  the  black  glass  and  mica. 

No  more  striking  or  instructive  illustration  of  the 
influence  of  coincidence  could  be  adduced  than  that 
furnished  by  the  radiation  from  a  carbonic  oxide  flame. 
Here  the  product  of  combustion  is  carbonic  acid;  and 
on  the  radiation  from  this  flame  even  the  ordinary 
carbonic  acid  of  the  atmosphere  exerts  a  powerful 
effect.  A  quantity  of  the  gas,  only  one-thirtieth  of  an 
atmosphere  in  density,  contained  in  a  polished  brass 
tube  four  feet  long,  intercepts  50  per  cent,  of  the 
radiation  from  the  carbonic  oxide  flame.  For  the  heat 
emitted  by  lampblack,  olefiant  gas  is  a  far  more  pow- 
erful absorber  than  carbonic  acid;  in  fact,  for  such 
heat,  with  one  exception,  carbonic  acid  is  the  most 
feeble  absorber  to  be  found  among  the  compound  gases. 
Moreover,  for  the  radiation  from  a  hydrogen  flame 
olefiant  gas  possesses  twice  the  absorbent  power  of 
carbonic  acid,  while  for  the  radiation  from  the  carbonic 
oxide  flame,  at  a  common  pressure  of  one  inch  of  mer- 
cury, the  absorption  by  carbonic  acid  is  more  than 
twice  that  of  olefiant  gas.  Thus  we  establish  the  co- 
incidence of  period  between  carbonic  acid  at  a  tem- 
perature of  20°  C.  and  carbonic  acid  at  a  temperature 
of  over  3000°  C.,  the  periods  of  oscillation  of  both  the 
incandescent  and  the  cold  gas  belonging  to  the  ultra- 
red  portion  of  the  spectrum. 

It  will  be  seen  from  the  foregoing  remarks  and 
experiments  how  impossible  it  is  to  determine  the  effect 
of  temperature  pure  and  simple  on  the  transmission  of 
radiant  heat  if  different  sources  of  heat  be  employed. 
Throughout  such  an  examination  the  same  oscillating 
atoms  ought  to  be  retained.  This  is  done  by  heating 
a  platinum  spiral  by  an  electric  current,  the  tempera- 


CONTRIBUTIONS    TO    MOLECULAR    PHYSICS.    395 

ture  meanwhile  varying  between  the  widest  possible 
limits.  Their  comparative  opacity  to  the  ultra-red 
rays  shows  the  general  accord  of  the  oscillating  periods 
of  the  vapours  referred  to  at  the  commencement  of  this 
lecture  with  those  of  the  ultra-red  undulations.  Hence, 
by  gradually  heating  a  platinum  wire  from  darkness 
up  to  whiteness,  we  ought  gradually  to  augment  the 
discord  between  it  and  these  vapours,  and  thus  aug- 
ment the  transmission.  Experiment  entirely  confirms 
this  conclusion.  Formic  ether,  for  example,  absorbs 
45  per  cent,  of  the  radiation  from  a  platinum  spiral 
heated  to  barely  visible  redness;  32  per  cent,  of  the 
radiation  from  the  same  spiral  at  a  red  heat;  26  per 
cent,  of  the  radiation  from  a  white-hot  spiral,  and  only 
21  per  cent,  when  the  spiral  is  brought  near  its  point 
of  fusion.  Remarkable  cases  of  inversion  as  to  trans- 
parency also  occur.  For  barely  visible  redness  formic 
ether  is  more  opaque  than  sulphuric;  for  a  bright  red 
heat  both  are  equally  transparent;  while,  for  a  white 
heat,  and  still  more  for  a  higher  temperature,  sulphuric 
ether  is  more  opaque  than  formic.  This  result  gives 
us  a  clear  view  of  the  relationship  of  the  two  substances 
to  the  luminiferous  ether.  As  we  introduce  waves  of 
shorter  period  the  sulphuric  ether  augments  most 
rapidly  in  opacity;  that  is  to  say,  its  accord  with  the 
shorter  waves  is  greater  than  that  of  the  formic.  Hence 
we  may  infer  that  the  atoms  of  formic  ether  oscillate, 
on  the  whole,  more  slowly  than  those  of  sulphuric 
ether. 

When  the  source  of  heat  is  a  Leslie's  cube  coated 
with  lampblack  and  filled  with  boiling  water,  the  opac- 
ity of  formic  ether  in  comparison  with  sulphuric  is 
very  decided.  With  this  source  also  the  positions  of 
chloroform  and  iodide  of  methyl  are  inverted.  For 
a  white-hot  spiral,  the  absorption  of  chloroform  vapour 


396  FKAGMENTS    OF    SCIENCE. 

being  10  per  cent.,  that  of  iodide  of  methyl  is  16;  with 
the  blackened  cube  as  source,  the  absorption  by  chloro- 
form is  22  per  cent.,  .while  that  by  the  iodide  of  methyl 
is  only  19.  This  inversion  is  not  the  result  of  tem- 
perature merely;  for  when  a  platinum  wire,  heated  to 
the  temperature  of  boiling  water,  is  employed  as  a 
source,  the  iodide  continues  to  be  the  most  powerful 
absorber.  All  the  experiments  hitherto  made  go  to 
prove  that  from  heated  lampblack  an  emission  takes 
place  which  synchronises  in  an  especial  manner  with 
chloroform.  For  the  cube  at  100°  C.,  coated  with 
lampblack,  the  absorption  by  chloroform  is  more  than 
three  times  that  by  sulphide  of  carbon;  for  the  radia- 
tion from  the  most  luminous  portion  of  a  gas-flame  the 
absorption  by  chloroform  is  also  considerably  in  excess 
of  that  by  bisulphide  of  carbon;  while,  for  the  flame 
of  a  Bunsen's  burner,  from  which  the  incandescent 
carbon  particles  are  removed  by  the  free  admixture  of 
air,  the  absorption  by  bisulphide  of  carbon  is  nearly 
twice  that  of  chloroform.  The  removal  of  the  carbon 
particles  more  than  doubles  the  relative  transparency  of 
the  chloroform.  Testing,  moreover,  the  radiation  from 
various  parts  of  the  same  flame,  it  was  found  that  for 
the  blue  base  of  the  flame  the  bisulphide  of  carbon  was 
most  opaque,  while  for  all  other  parts  of  the  flame 
the  chloroform  was  most  opaque.  For  the  radiation 
from  a  very  small  gas  flame,  consisting  of  a  blue  base 
and  a  small  white  tip,  the  bisulphide  was  also  most 
opaque,  and  its  opacity  very  decidedly  exceeded  that 
of  the  chloroform  when  the  source  of  heat  was  the 
flame  of  bisulphide  of  carbon.  Comparing  the  radia- 
tion from  a  Leslie's  cube  coated  with  isinglass  with 
that  from  a  similar  cube  coated  with  lampblack,  at  the 
common  temperature  of  100°  C.,  it  was  found  that, 
out  of  eleven  vapours,  all  but  one  absorbed  the  radia- 


CONTRIBUTIONS    TO    MOLECULAB    PHYSICS.    397 

tion  from  the  isinglass  most  powerfully;  the  single 
exception  was  chloroform. 

It  is  worthy  of  remark  that  whenever,  through  a 
change  of  source,  the  position  of  a  vapour  as  an  ab- 
sorber of  radiant  heat  was  altered,  the  position  of  the 
liquid  from  which  the  vapour  was  derived  underwent 
a  similar  change. 

It  is  still  a  point  of  difference  between  eminent 
investigators  whether  radiant  heat,  up  to  a  temperature 
of  100°  €.,  is  monochromatic  or  not.  Some  affirm  this; 
some  deny  it.  A  long  series  of  experiments  enables 
me  to  state  that  probably  no  two  substances  at  a  tem- 
perature of  100°  C.  emit  heat  of  the  same  quality. 
The  heat  emitted  by  isinglass,  for  example,  is  different 
from  that  emitted  by  lampblack,  and  the  heat  emitted 
by  cloth,  or  paper,  differs  from  both.  It  is  also  a  sub- 
ject of  discussion  whether  rock-salt  is  equally  dia- 
thermic to  all  kinds  of  calorific  rays;  the  differences 
affirmed  to  exist  by  some  investigators  being  ascribed 
by  others  to  differences  of  incidence  from  the  various 
sources  employed.  MM.  de  la  Provostaye  and  Desains 
maintain  the  former  view,  Melloni  and  M.  Knoblauch 
maintain  the  latter.  I  tested  this  point  without 
changing  anything  but  the  temperature  of  the  source; 
its  size,  distance,  and  surroundings  remaining  the  same. 
The  experiments  proved  rock-salt  to  be  coloured  ther- 
mally. It  is  more  opaque,  for  example,  to  the  radiation 
from  a  barely  visible  spiral  than  to  that  from  a  white- 
hot  one. 

In  regard  to  the  relation  of  radiation  to  conduction, 
if  we  define  radiation,  internal  as  well  as  external,  as 
the  communication  of  motion  from  the  vibrating  atoms 
to  the  ether,  we  may,  I  think,  by  fair  theoretic  reason- 
ing, reach  the  conclusion  that  the  best  radiators  ought 
to  prove  the  worst  conductors.  A  broad  consideration 


398  FRAGMENTS    OF    SCIENCE. 

of  the  subject  shows  at  once  the  general  harmony  of 
this  conclusion  with  observed  facts.  Organic  sub- 
stances are  all  excellent  radiators;  they  are  also  ex- 
tremely bad  conductors.  The  moment  we  pass  from 
the  metals  to  their  compounds  we  pass  from  good  con- 
ductors to  bad  ones,  and  from  bad  radiators  to  good 
ones.  Water,  among  liquids,  is  probably  the  worst  con- 
ductor; it  is  the  best  radiator.  Silver,  among  solids, 
is  the  best  conductor;  it  is  the  worst  radiator.  The 
excellent  researches  of  MM.  de  la  Provostaye  and  De- 
sains  furnish  a  striking  illustration  of  what  I  am  in- 
clined to  regard  as  a  natural  law — that  those  atoms 
which  transfer  the  greatest  amount  of  motion  to  the 
ether,  or,  in  other  words,  radiate  most  powerfully,  are 
the  least  competent  to  communicate  motion  to  each 
other,  or,  in  other  words,  to  propagate  by  conduction 
readily. 


XVIII. 

LIFE  AND  LETTERS  OF  FARADAY. 
1870. 

YTNDERTAKEN  and  executed  in  a  reverent  and 
U  loving  spirit,  the  work  of  Dr.  Bence  Jones 
makes  Faraday  the  virtual  writer  of  his  own  life. 
Everybody  now  knows  the  story  of  the  philosopher's 
birth;  that  his  father  was  a  smith;  that  he  was  born 
at  Newington  Butts  in  1791;  that  he  ran  along  the 
London  pavements,  a  bright-eyed  errand  boy,  with  a 
load  of  brown  curls  upon  his  head  and  a  packet  of 
newspapers  under  his  arm;  that  the  lad's  master  was  a 
bookseller  and  bookbinder — a  kindly  man,  who  became 
attached  to  the  little  fellow,  and  in  due  time  made  him 
his  apprentice  without  fee;  that  during  his  apprentice- 
ship he  found  his  appetite  for  knowledge  provoked  and 
strengthened  by  the  books  he  stitched  and  covered. 
Thus  he  grew  in  wisdom  and  stature  to  his  year  of  legal 
manhood,  when  he  appears  in  the  volumes  before  us 
as  a  writer  of  letters,  which  reveal  his  occupation,  ac- 
quirements, and  tone  of  mind.  His  correspondent  was 
Mr.  Abbott,  a  member  of  the  Society  of  Friends,  who, 
with  a  forecast  of  his  correspondent's  greatness,  pre- 
served his  letters  and  produced  them  at  the  proper  time. 
In  later  years  Faraday  always  carried  in  his  pocket 
a  blank  card,  on  which  he  jotted  down  in  pencil  his 
thoughts  and  memoranda.  He  made  his  notes  in  the 
laboratory,  in  the  theatre,  and  in  the  streets.  This 


400  FKAGMENTS    OF    SCIENCE. 

distrust  of  his  memory  reveals  itself  in  his  first  letter 
to  Abbott.  To  a  proposition  that  no  new  enquiry 
should  be  started  between  them  before  the  old  one  had 
been  exhaustively,  discussed,  Faraday  objects.  '  Your 
notion/  he  says,  '  I  can  hardly  allow,  for  the  following 
reason:  ideas  and  thoughts  spring  up  in  my  mind 
which  are  irrevocably  lost  for  want  of  noting  at  the 
time.'  Gentle  as  he  seemed,  he  wished  to  have  his  own 
way,  and  he  had  it  throughout  his  life.  Differences  of 
opinion  sometimes  arose  between  the  two  friends,  and 
then  they  resolutely  faced  each  other.  '  I  accept  your 
offer  to  fight  it  out  with  joy,  and  shall  in  the  battle  of 
experience  cause  not  pain,  but,  I  hope,  pleasure/ 
Faraday  notes  his  own  impetuosity,  and  incessantly 
checks  it.  There  is  at  times  something  almost  mechan- 
ical in  his  self-restraint.  In  another  nature  it  would 
have  hardened  into  mere  '  correctness '  of  conduct; 
but  his  overflowing  affections  prevented  this  in  his 
case.  The  habit  of  self-control  became  a  second  nature 
to  him  at  last,  and  lent  serenity  to  his  later  years. 

In  October,  1812,  he  was  engaged  by  a  Mr.  De  la 
Eoche  as  a  journeyman  bookbinder;  but  the  situation 
did  not  suit  him.  His  master  appears  to  have  been  an 
austere  and  passionate  man,  and  Faraday  was  to  the 
last  degree  sensitive.  All  his  life  he  continued  so.  He 
suffered  at  times  from  dejection;  and  a  certain  grim- 
ness,  too,  pervaded  his  moods.  '  At  present/  he  writes 
to  Abbott,  '  I  am  as  serious  as  you  can  be,  and  would 
not  scruple  to  speak  a  truth  to  any  human  being,  what- 
ever repugnance  it  might  give  rise  to.  Being  in  this 
state  of  mind,  I  should  have  refrained  from  writing  to 
you,  did  I  not  conceive  from  the  general  tenor  of  your 
letters  that  your  mind  is,  at  proper  times,  occupied  upon 
serious  subjects  to  the  exclusion  of  those  that  are 
frivolous/  Plainly  he  had  fallen  into  that  stern  Puri- 


FARADAY.  401 

tan  mood,  which  not  only  crucifies  the  affections  and 
lusts  of  him  who  harbours  it,  but  is  often  a  cause  of 
disturbed  digestion  to  his  friends. 

About  three  months  after  his  engagement  with 
De  la  Roche,  Faraday  quitted  him  and  bookbinding 
together.  He  had  heard  Davy,  copied  his  lectures,  and 
written  to  him,  entreating  to  be  released  from  Trade, 
which  he  hated,  and  enabled  to  pursue  Science.  Davy 
recognised  the  merit  of  his  correspondent,  kept  his  eye 
upon  him,  and,  when  occasion  offered,  drove  to  his 
door  and  sent  in  a  letter,  offering  him  the  post  of  assist- 
ant in  the  laboratory  of  the  Royal  Institution.  He  was 
engaged  March  1,  1813,  and  on  the  8th  we  find  him 
extracting  the  sugar  from  beet-root.  He  joined  the 
City  Philosophical  Society  which  had  been  founded  by 
Mr.  Tatum  in  1808.  '  The  discipline  was  very  sturdy, 
the  remarks  very  plain,  and  the  results  most  valuable.' 
Faraday  derived  great  profit  from  this  little  association. 
In  the  laboratory  he  had  a  discipline  sturdier  still. 
Both  Davy  and  himself  were  at  this  time  frequently  cut 
and  bruised  by  explosions  of  chloride  of  nitrogen.  One 
explosion  was  so  rapid  '  as  to  blow  my  hand  open,  tear 
away  a  part  of  one  nail,  and  make  my  fingers  so  sore 
that  I  cannot  use  them  easily/  In  another  experiment 
'  the  tube  and  receiver  were  blown  to  pieces,  I  got  a  cut 
on  the  head,  and  Sir  Humphry  a  bruise  on  his  hand.' 
And  again  speaking  of  the  same  substance,  he  says, 
'  when  put  in  the  pump  and  exhausted,  it  stood  for  a 
moment,  and  then  exploded  with  a  fearful  noise.  Both 
Sir  H.  and  I  had  masks  on,  but  I  escaped  this  time  the 
best.  Sir  H.  had  his  face  cut  in  two  places  about  the 
chin,  and  a  violent  blow  on  the  forehead  struck  through 
a  considerable  thickness  of  silk  and  leather/  It  was 
this  same  substance  that  blew  out  the  eye  of  Dulong. 

Over  and  over  again,  even  at  this  early  date,  we  can 


402  FRAGMENTS    OF    SCIENCE. 

discern  the  quality  which,  compounded  with  his  rare 
intellectual  power,  made  Faraday  a  great  experimental 
philosopher.  This  was  his  desire  to  see  facts,  and  not 
to  rest  contented  with  the  descriptions  of  them.  He 
frequently  pits  the  eye  against  the  ear,  and  affirms  the 
enormous  superiority  of  the  organ  of  vision.  Late  in 
life  I  have  heard  him  say  that  he  could  never  fully 
understand  an  experiment  until  he  had  seen  it.  But 
he  did  not  confine  himself  to  experiment.  He  aspired 
to  be  a  teacher,  and  reflected  and  wrote  upon  the 
method  of  scientific  exposition.  '  A  lecturer,'  he  ob- 
serves, '  should  appear  easy  and  collected,  undaunted 
and  unconcerned:'  still  'his  whole  behaviour  should 
evince  respect  for  his  audience.'  These  recommenda- 
tions were  afterwards  in  great  part  embodied  by  him- 
self. I  doubt  his  '  unconcern,'  but  his  fearlessness  was 
often  manifested.  It  used  to  rise  within  him  as  a  wave, 
which  carried  both  him  and  his  audience  along  with 
it.  On  rare  occasions  also,  when  he  felt  himself  and 
his  subject  hopelessly  unintelligible,  he  suddenly 
evoked  a  certain  recklessness  of  thought,  and,  without 
halting  to  extricate  his  bewildered  followers,  he  would 
dash  alone  through  the  jungle  into  which  he  had  un- 
wittingly led  them;  thus  saving  them  from  ennui  by 
the  exhibition  of  a  vigour  which,  for  the  time  being, 
they  could  neither  share  nor  comprehend. 

In  October,  1813,  he  quitted  England  with  Sir 
Humphry  and  Lady  Davy.  During  his  absence  he  kept 
a  journal,  from  which  copious  and  interesting  extracts 
have  been  made  by  Dr.  Bence  Jones.  Davy  was  con- 
siderate, preferring  at  times  to  be  his  own  servant 
rather  than  impose  on  Faraday  duties  which  he  dis- 
liked. But  Lady  Davy  was  the  reverse.  She  treated 
him  as  an  underling;  he  chafed  under  the  treatment, 
and  was  often  on  the  point  of  returning  home.  They 


FARADAY.  403 

halted  at  Geneva.  De  la  Rive,  the  elder,  had  known 
Davy  in  1799,  and,  by  his  writings  in  the  '  Bibliotheque 
Britannique,'  had  been  the  first  to  make  the  English 
chemist's  labours  known  abroad.  He  welcomed  Davy 
to  his  country  residence  in  1814.  Both  were  sports- 
men, and  they  often  went  out  shooting  together.  On 
these  occasions  Faraday  charged  Davy's  gun  while  De 
la  Rive  charged  his  own.  Once  the  Genevese  philoso- 
pher found  himself  by  the  side  of  Faraday,  and  in  his 
frank  and  genial  way  entered  into  conversation  with 
the  young  man.  It  was  evident  that  a  person  possess- 
ing such  a  charm  of  manner  and  such  high  intelligence 
could  be  no  mere  servant.  On  enquiry  De  la  Rive  was 
somewhat  shocked  to  find  that  the  soi-disant  domes- 
tique  was  really  preparateur  in  the  laboratory  of  the 
Royal  Institution;  and  he  immediately  proposed  that 
Faraday  thenceforth  should  join  the  masters  instead 
of  the  servants  at  their  meals.  To  this  Davy,  probably 
out  of  weak  deference  to  his  wife,  objected;  but  an  ar- 
rangement was  come  to  that  Faraday  thenceforward 
should  have  his  food  in  his  own  room.  Rumour  states 
that  a  dinner  in  honour  of  Faraday  was  given  by  De  la 
Rive.  This  is  a  delusion;  there  was  no  such  banquet; 
but  Faraday  never  forgot  the  kindness  of  the  friend  who 
saw  his  merit  when  he  was  a  mere  gargon  de  Idbora- 
toire.* 

He  returned  in  1815  to  the  Royal  Institution.  Here 
he  helped  Davy  for  years;  he  worked  also  for  himself, 

*  While  confined  last  autumn  at  Geneva  by  the  effects  of  a 
fall  in  the  Alps,  my  friends,  with  a  kindness  I  can  never  forget, 
did  all  that  friendship  could  suggest  to  render  my  captivity 
pleasant  to  me.  M.  de  la  Rive  then  wrote  out  for  me  the  full 
account,  of  which  the  foregoing  is  a  condensed  abstract.  It  was 
at  the  desire  of  Dr.  Bence  Jones  that  I  asked  him  to  do  so.  The 
rumour  of  a  banquet  at  Geneva  illustrates  the  tendency  to  sub- 
stitute for  the  youth  of  1814  the  Faraday  of  later  years. 


404  FKAGMENTS    OF    SCIENCE. 

and  lectured  frequently  at  the  City  Philosophical  So- 
ciety. He  took  lessons  in  elocution,  happily  without 
damage  to  his  natural  force,  earnestness,  and  grace  of 
delivery.  He  was  never  pledged  to  theory,  and  he 
changed  in  opinion  as  knowledge  advanced.  With  him 
life  was  growth.  In  those  early  lectures  we  hear  him 
say,  ( In  knowledge,  that  man  only  is  to  be  contemned 
and  despised  who  is  not  in  a  state  of  transition.'  And 
again:  '  Nothing  is  more  difficult  and  requires  more 
caution  than  philosophical  deduction,  nor  is  there  any- 
thing more  adverse  to  its  accuracy  than  fixity  of  opin- 
ion/ Not  that  he  was  wafted  about  by  every  wind  of 
doctrine;  but  that  he  united  flexibility  with  his 
strength.  In  striking  contrast  with  this  intellectual 
expansiveness  was  his  fixity  in  religion,  but  this  is  a 
subject  which  cannot  be  discussed  here. 

Of  all  the  letters  published  in  these  volumes  none 
possess  a  greater  charm  than  those  of  Faraday  to  his 
wife.  Here,  as  Dr.  Bence  Jones  truly  remarks, '  he  laid 
open  all  his  mind  and  the  whole  of  his  character,  and 
what  can  be  made  known  can  scarcely  fail  to  charm 
every  one  by  its  loveliness,  its  truthfulness,  and  its  ear- 
nestness.' Abbott  and  he  sometimes  swerved  into  word- 
play about  love;  but  up  to  1820,  or  thereabouts,  the 
passion  was  potential  merely.  Faraday's  journal  indeed 
contains  entries  which  show  that  he  took  pleasure  in 
the  assertion  of  his  contempt  for  love;  but  these  very 
entries  became  links  in  his  destiny.  It  was  through 
them  that  he  became  acquainted  with  one  who  inspired 
him  with  a  feeling  which  only  ended  with  his  life.  His 
biographer  has  given  us  the  means  of  tracing  the  vary- 
ing moods  which  preceded  his  acceptance.  They  re- 
veal more  than  the  common  alternations  of  light  and 
gloom;  at  one  moment  he  wishes  that  his  flesh  might 
melt  and  that  he  might  become  nothing;  at  another  he 


FARADAY.  405 

is  intoxicated  with  hope.  The  impetuosity  of  his  char- 
acter was  then  unchastened  by  the  discipline  to  which 
it  was  subjected  in  after  years.  The  very  strength  of 
his  passion  proved  for  a  time  a  bar  to  its  advance,  sug- 
gesting, as  it  did,  to  the  conscientious  mind  of  Miss 
Barnard,  doubts  of  her  capability  to  return  it  with  ade- 
quate force.  But  they  met  again  and  again,  and  at  each 
successive  meeting  he  found  his  heaven  clearer,  until 
at  length  he  was  able  to  say,  '  Not  a  moment's  alloy  of 
this  evening's  happiness  occurred.  Everything  was 
delightful  to  the  last  moment  of  my  stay  with  my  com- 
panion, because  she  was  so/  The  turbulence  of  doubt 
subsided,  and  a  calm  and  elevating  confidence  took  its 
place.  '  What  can  I  call  myself/  he  writes  to  her  in  a 
subsequent  letter,  *  to  convey  most  perfectly  my  af- 
fection and  love  for  you?  Can  I  or  can  truth  say  more 
than  that  for  this  world  I  am  yours? '  Assuredly  he 
made  his  profession  good,  and  no  fairer  light  falls  upon 
his  character  than  that  which  reveals  his  relations  to 
his  wife.  Never,  I  believe,  existed  a  manlier,  purer, 
steadier  love.  Like  a  burning  diamond,  it  continued 
to  shed,  for  six-and-forty  years,  its  white  and  smoke- 
less glow. 

Faraday  was  married  on  June  12,  1821;  and  up  to 
this  date  Davy  appears  throughout  as  his  friend.  Soon 
afterwards,  however,  disunion  occurred  between  them, 
which,  while  it  lasted,  must  have  given  Faraday  intense 
pain.  It  is  impossible  to  doubt  the  honesty  of  con- 
viction with  which  this  subject  has  been  treated  by 
Dr.  Bence  Jones,  and  there  may  be  facts  known  to  him, 
but  not  appearing  in  these  volumes,  which  justify  his 
opinion  that  Davy  in  those  days  had  become  jealous  of 
Faraday.  This,  which  is  the  prevalent  belief,  is  also 
reproduced  in  an  excellent  article  in  the  March  num- 
ber of  '  Fraser's  Magazine.'  But  the  best  analysis  I  can 


406  FKAGMENTS    OF    SCIENCE. 

make  of  the  data  fails  to  present  Davy  in  this  light  to 
me.  The  facts,  as  I  regard  them,  are  briefly  these. 

In  1820,  Oersted  of  Copenhagen  made  the  cele- 
brated discovery  which  connects  electricity  with  mag- 
netism, and  immediately  afterwards  the  acute  mind  of 
Wollaston  perceived  that  a  wire  carrying  a  current 
ought  to  rotate  round  its  own  axis  under  the  influence 
of  a  magnetic  pole.  In  1821  he  tried,  but  failed,  to 
realise  this  result  in  the  laboratory  of  the  Eoyal  Insti- 
tution. Faraday  was  not  present  at  the  moment,  but 
he  came  in  immediately  afterwards  and  heard  the  con- 
versation of  Wollaston  and  Davy  about  the  experiment. 
He  had  also  heard  a  rumour  of  a  wager  that  Dr.  Wol- 
laston would  eventually  succeed. 

This  was  in  April.  In  the  autumn  of  the  same 
year  Faraday  wrote  a  history  of  electro-magnetism, 
and  repeated  for  himself  the  experiments  which  he 
described.  It  was  while  thus  instructing  himself  that 
he  succeeded  in  causing  a  wire,  carrying  an  electric 
current,  to  rotate  round  a  magnetic  pole.  This  was 
not  the  result  sought  by  Wollaston,  but  it  was  closely 
related  to  that  result. 

The  strong  tendency  of  Faraday's  mind  to  look 
upon  the  reciprocal  actions  of  natural  forces  gave  birth 
to  his  greatest  discoveries;  and  we,  who  know  this, 
should  be  justified  in  concluding  that,  even  had  Wol- 
laston not  preceded  him,  the  result  would  have  been 
the  same.  But  in  judging  Davy  we  ought  to  transport 
ourselves  to  his  time,  and  carefully  exclude  from  our 
thoughts  and  feelings  that  noble  subsequent  life,  which 
would  render  simply  impossible  the  ascription  to  Fara- 
day of  anything  unfair.  It  would  be  unjust  to  Davy  to 
put  our  knowledge  in  the  place  of  his,  or  to  credit  him 
with  data  which  he  could  not  have  possessed.  Rumour 
and  fact  had  connected  the  name  of  Wollaston  with 


FARADAY.  407 

these  supposed  interactions  between  magnets  and  cur- 
rents. When,  therefore,  Faraday  in  October  published 
his  successful  experiment,  without  any  allusion  to  Wol- 
laston,  general,  though  really  ungrounded,  criticism 
followed.  I  say  ungrounded  because,  firstly,  Faraday's 
experiment  was  not  that  of  Wollaston,  and  secondly, 
Faraday,  before  he  published  it,  had  actually  called 
upon  Wollaston,  and  not  finding  him  at  home,  did  not 
feel  himself  authorised  to  mention  his  name. 

In  December,  Faraday  published  a  second  paper  on 
the  same  subject,  from  which,  through  a  misappre- 
hension, the  name  of  Wollaston  was  also  omitted. 
Warburton  and  others  thereupon  affirmed  that  Wol- 
laston's  ideas  had  been  appropriated  without  acknowl- 
edgment, and  it  is  plain  that  Wollaston  himself,  though 
cautious  in  his  utterance,  was  also  hurt.  Censure 
grew  till  it  became  intolerable.  '  I  hear/  writes  Fara- 
day to  his  friend  Stodart,  '  every  day  more  and  more 
of  these  sounds,  which,  though  only  whispers  to  me, 
are,  I  suspect,  spoken  aloud  among  scientific  men.' 
He  might  have  written  explanations  and  defences,  but 
he  went  straighter  to  the  point.  He  wished  to  see 
the  principals  face  to  face — to  plead  his  cause  before 
them  personally.  There  was  a  certain  vehemence  in 
his  desire  to  do  this.  He  saw  Wollaston,  he  saw  Davy, 
he  saw  Warburton;  and  I  am  inclined  to  think  that 
it  was  the  irresistible  candour  and  truth  of  character 
which  these  viva  voce  defences  revealed,  as  much  as 
the  defences  themselves,  that  disarmed  resentment  at 
the  time. 

As  regards  Davy,  another  cause  of  dissension  arose 
in  1823.  In  the  spring  of  that  year  Faraday  analysed 
the  hydrate  of  chlorine,  a  substance  once  believed  to  be 
the  element  chlorine,  but  proved  by  Davy  to  be  a 
compound  of  that  element  and  water.  The  analysis 
27 


408  FKAGMENTS    OF    SCIENCE. 

was  looked  over  by  Davy,  who  then  and  there  sug- 
gested to  Faraday  to  heat  the  hydrate  in  a  closed  glass 
tube.  This  was  done,  the  substance  was  decomposed, 
and  one  of  the  products  of  decomposition  was  proved 
by  Faraday  to  be  chlorine  liquefied  by  its  own  pressure. 
On  the  day  of  its  discovery  he  communicated  this  re- 
sult to  Dr.  Paris.  Davy,  on  being  informed  of  it,  in- 
stantly liquefied  another  gas  in  the  same  way.  Having 
struck  thus  into  Faraday's  enquiry,  ought  he  not  to 
have  left  the  matter  in  Faraday's  hands?  I  think  he 
ought.  But,  considering  his  relation  to  both  Faraday 
and  the  hydrate  of  chlorine,  Davy,  I  submit,  may  be 
excused  for  thinking  differently.  A  father  is  not  al- 
ways wise  enough  to  see  that  his  son  has  ceased  to  be 
a  boy,  and  estrangement  on  this  account  is  not  rare; 
nor  was  Davy  wise  enough  to  discern  that  Faraday  had 
passed  the  mere  assistant  stage,  and  become  a  discov- 
erer. It  is  now  hard  to  avoid  magnifying  this  error. 
But  had  Faraday  died  or  ceased  to  work  at  this  time, 
or  had  his  subsequent  life  been  devoted  to  money-get- 
ting, instead  of  to  research,  would  anybody  now  dream 
of  ascribing  jealousy  to  Davy?  Assuredly  not.  Why 
should  he  be  jealous?  His  reputation  at  this  time  was 
almost  without  a  parallel:  his  glory  was  without  a 
cloud.  He  had  added  to  his  other  discoveries  that  of 
Faraday,  and  after  having  been  his  teacher  for  seven 
years,  his  language  to  him  was  this:  '  It  gives  me  great 
pleasure  to  hear  that  you  are  comfortable  at  the  Eoyal 
Institution,  and  I  trust  that  you  will  not  only  do  some- 
thing good  and  honourable  for  yourself,  but  also  for 
science/  This  is  not  the  language  of  jealousy,  poten- 
tial or  actual.  But  the  chlorine  business  introduced 
irritation  and  anger,  to  which,  and  not  to  any  ignobler 
motive,  Davy's  opposition  to  the  election  of  Faraday  to 
the  Eoyal  Society  is,  I  am  persuaded,  to  be  ascribed. 


FARADAY.  409 

These  matters  are  touched  upon  with  perfect  can- 
dour, and  becoming  consideration,  in  the  volumes  of 
Dr.  Bence  Jones;  but  in  '  society '  they  are  not  always 
so  handled.  Here  a  name  of  noble  intellectual  associ- 
ations is  surrounded  by  injurious  rumours  which  I 
would  willingly  scatter  for  ever.  The  pupil's  magni- 
tude, and  the  splendour  of  his  position,  are  too  great 
and  absolute  to  need  as  a  foil  the  humiliation  of  his 
master.  Brothers  in  intellect,  Davy  and  Faraday,  how- 
ever, could  never  have  become  brothers  in  feeling;  their 
characters  were  too  unlike.  Davy  loved  the  pomp  and 
circumstance  of  fame;  Faraday  the  inner  consciousness 
that  he  had  fairly  won  renown.  They  were  both  proud 
men.  But  with  Davy  pride  projected  itself  into  the 
outer  world;  while  with  Faraday  it  became  a  steadying 
and  dignifying  inward  force.  In  one  great  particular 
they  agreed.  Each  of  them  could  have  turned  his  sci- 
ence to  immense  commercial  profit,  but  neither  of 
them  did  so.  The  noble  excitement  of  research,  and 
the  delight  of  discovery,  constituted  their  reward.  I 
commend  them  to  the  reverence  which  great  gifts 
greatly  exercised  ought  to  inspire.  They  were  both 
ours;  and  through  the  coming  centuries  England  will 
be  able  to  point  with  just  pride  to  the  possession  of 
such  men. 


The  first  volume  of  the  '  Life  and  Letters '  reveals 
to  us  the  youth  who  was  to  be  father  to  the  man. 
Skilful,  aspiring,  resolute,  he  grew  steadily  in  knowl- 
edge and  in  power.  Consciously  or  unconsciously,  the 
relation  of  Action  to  Reaction  was  ever  present  to 
Faraday's  mind.  It  had  been  fostered  by  his  discovery 
of  Magnetic  Rotations,  and  it  planted  in  him  more 


410  FEAGMENTS    OF    SCIENCE. 

daring  ideas  of  a  similar  kind.  Magnetism  he  knew 
could  be  evoked  by  electricity,  and  he  thought  that 
electricity,  in  its  turn,  ought  to  be  capable  of  evolution 
by  magnetism.  On  August  29,  1831,  his  experiments 
on  this  subject  began.  He  had  been  fortified  by  pre- 
vious trials,  which,  though  failures,  had  begotten  in- 
stincts directing  him  towards  the  truth.  He,  like 
every  strong  worker,  might  at  times  miss  the  outward 
object,,  but  he  always  gained  the  inner  light,  education, 
and  expansion.  Of  this  Faraday's  life  was  a  constant 
illustration.  By  November  he  had  discovered  and  col- 
ligated a  multitude  of  the  most  wonderful  and  unex- 
pected phenomena.  He  had  generated  currents  by 
currents;  currents  by  magnets,  permanent  and  transi- 
tory; and  he  afterwards  generated  currents  by  the 
earth  itself.  Arago's  '  Magnetism  of  Eotation/  which 
had  for  years  offered  itself  as  a  challenge  to  the  best 
scientific  intellects  of  Europe,  now  fell  into  his  hands. 
It  proved  to  be  a  beautiful,  but  still  special,  illustration 
of  the  great  principle  of  Magneto-electric  Induction. 
Nothing  equal  to  this  latter,  in  the  way  of  pure  experi- 
mental enquiry,  had  previously  been  achieved. 

Electricities  from  various  sources  were  next  exam- 
ined, and  their  differences  and  resemblances  revealed. 
He  thus  assured  himself  of  their  substantial  identity. 
He  then  took  up  Conduction,  and  gave  many  striking 
illustrations  of  the  influence  of  Fusion  on  Conducting 
Power.  Eenouncing  professional  work,  from  which  at 
this  time  he  might  have  derived  an  income  of  many 
thousands  a  year,  he  poured  his  whole  momentum  into 
his  researches.  He  was  long  entangled  in  Electro- 
chemistry. The  light  of  law  was  for  a  time  obscured 
by  the  thick  umbrage  of  novel  facts;  but  he  finally 
emerged  from  his  researches  with  the  great  principle  of 
Definite  Electro-chemical  Decomposition  in  his  hands. 


FARADAY.  411 

If  his  discovery  of  Magneto-electricity  may  be  ranked 
with  that  of  the  pile  by  Volta,  this  new  discovery 
may  almost  stand  beside  that  of  Definite  Combining 
Proportions  in  Chemistry.  He  passed  on  to  Static 
Electricity— its  Conduction,  Induction,  and  Mode  of 
Propagation.  He  discovered  and  illustrated  the  prin- 
ciple of  Inductive  Capacity;  and,  turning  to  theory, 
he  asked  himself  how  electrical  attractions  and  repul- 
sions are  transmitted.  Are  they,  like  gravity,  actions 
at  a  distance,  or  do  they  require  a  medium?  If  the 
former,  then,  like  gravity,  they  will  act  in  straight 
lines;  if  the  latter,  then,  like  sound  or  light,  they  may 
turn  a  corner.  Faraday  held — and  his  views  are  gain- 
ing ground — that  his  experiments  proved  the  fact  of 
curvilinear  propagation,  and  hence  the  operation  of  a 
medium.  Others  denied  this;  but  none  can  deny  the 
profound  and  philosophic  character  of  his  leading 
thought.*  The  first  volume  of  the  Researches  contains 
all  the  papers  here  referred  to. 

Faraday  had  heard  it  stated  that  henceforth  phys- 
ical discoveries  would  be  made  solely  by  the  aid  of 
mathematics;  that  we  had  our  data,  and  needed  only 
to  work  deductively.  Statements  of  a  similar  character 
crop  out  from  time  to  time  in  our  day.  They  arise 
from  an  imperfect  acquaintance  with  the  nature,  pres- 
ent condition,  and  prospective  vastness  of  the  field 
of  physical  enquiry.  The  tendency  of  natural  science 
doubtless  is  to  bring  all  physical  phenomena  under  the 
dominion  of  mechanical  laws;  to  give  them,  in  other 
words,  mathematical  expression.  But  our  approach  to 
this  result  is  asymptotic;  and  for  ages  to  come — pos- 
sibly for  all  the  ages  of  the  human  race — Nature  will 

*  In  a  very  remarkable  paper  published  in  PoggendorfFs '  An- 
nalen '  for  1857,  Werner  Siemens  accepts  and  develops  Faraday's 
theory  of  Molecular  Induction. 


412  FEAGMENTS    OF    SCIENCE. 

find  room  for  both  the  philosophical  experimenter  and 
the  mathematician.  Faraday  entered  his  protest  against 
the  foregoing  statement  by  labelling  his  investigations 
'  Experimental  Eesearches  in  Electricity.'  They  were 
completed  in  1854,  and  three  volumes  of  them  have 
been  published.  For  the  sake  of  reference,  he  num- 
bered every  paragraph,  the  last  number  being  3,362. 
In  1859  he  collected  and  published  a  fourth  volume 
of  papers,  under  the  title,  'Experimental  Eesearches 
in  Chemistry  and  Physics.'  Thus  did  this  apostle  of 
experiment  illustrate  its  power,  and  magnify  his  office. 

The  second  volume  of  the  Researches  embraces 
memoirs  on  the  Electricity  of  the  Gymnotus;  on  the 
Source  of  Power  in  the  Voltaic  Pile;  on  the  Electricity 
evolved  by  the  Friction  of  Water  and  Steam,  in  which 
the  phenomena  and  principles  of  Sir  William  Arm- 
strong's Hydro-electric  machine  are  described  and  de- 
veloped; a  paper  on  Magnetic  Rotations,  and  Faraday's 
letters  in  relation  to  the  controversy  it  aroused.  The 
contribution  of  most  permanent  value  here,  is  that  on 
the  Source  of  Power  in  the  Voltaic  Pile.  By  it  the 
Contact  Theory,  pure  and  simple,  was  totally  over- 
thrown, and  the  necessity  of  chemical  action  to  the 
maintenance  of  the  current  demonstrated. 

The  third  volume  of  the  Researches  opens  with  a 
memoir  entitled  '  The  Magnetisation  of  Light,'  and  the 
'  Illumination  of  Magnetic  Lines  of  Force.'  It  is  diffi- 
cult even  now  to  affix  a  definite  meaning  to  this  title; 
but  the  discovery  of  the  rotation  of  the  plane  of  polari- 
sation, which  it  announced,  seems  pregnant  with  great 
results.  The  writings  of  William  Thomson  on  the 
theoretic  aspects  of  the  discovery;  the  excellent  electro- 
dynamic  measurements  of  Wilhelm  Weber,  which  are 
models  of  experimental  completeness  and  skill;  Weber's 
labours  in  conjunction  with  his  lamented  friend  Kohl- 


FAEADAY.  413 

rausch — above  all,  the  researches  of  Clerk  Maxwell  on 
the  Electro-magnetic  Theory  of  Light — point  to  that 
wonderful  and  mysterious  medium,  which  is  the  vehicle 
of  light  and  radiant  heat,  as  the  probable  basis  also  of 
magnetic  and  electric  phenomena.  The  hope  of  such  a 
connection  was  first  raised  by  the  discovery  here  re- 
ferred to.*  Faraday  himself  seemed  to  cling  with  par- 
ticular affection  to  this  discovery.  He  felt  that  there 
was  more  in  it  than  he  was  able  to  unfold.  He  pre- 
dicted that  it  would  grow  in  meaning  with  the  growth 
of  science.  This  it  has  done;  this  it  is  doing  now. 
Its  right  interpretation  will  probably  mark  an  epoch 
in  scientific  history. 

Rapidly  following  it  is  the  discovery  of  Diamag- 
netism,  or  the  repulsion  of  matter  by  a  magnet.  Brug- 
mans  had  shown  that  bismuth  repelled  a  magnetic 
needle.  Here  he  stopped.  Le  Bailliff  proved  that 
antimony  did  the  same.  Here  he  stopped.  Seebeck, 
Becquerel,  and  others,  also  touched  the  discovery. 
These  fragmentary  gleams  excited  a  momentary  curi- 
osity and  were  almost  forgotten,  when  Faraday  inde- 
pendently alighted  on  the  same  facts;  and,  instead  of 
stopping,  made  them  the  inlets  to  a  new  and  vast  re- 
gion of  research.  The  value  of  a  discovery  is  to  be 
measured  by  the  intellectual  action  it  calls  forth;  and 

*  A  letter  addressed  to  me  by  Professor  Weber  on  March 
18  last  contains  the  following  reference  to  the  connection  here 
mentioned :  '  Die  Hoffnung  einer  solchen  Combination  ist  durch 
Faraday's  Entdeckung  der  Drehung  der  Polarisationsebene  durch 
inagnetische  Directionskraft  zuerst,  und  sodann  durch  die  Ueber- 
einstitnmung  derjenigen  Geschwindigkeit,  welche  das  Verh&ltniss 
derelectro-dynarnischen  Einbeit  zur  electro-Ftatisclien  ausdruckt, 
mit  der  Geschwindigkeit  des  Lichts  angeregt  wnrden  ;  und  mir 
scheint  von  alien  Versuchen,  welche  zur  Verwirklichung  dieser 
Hoffnung  gemacht  worden  sind,  das  von  Herrn  Maxwell  gemachte 
am  erfolgreichsten.' 


414  FEAGMENTS    OF    SCIENCE. 

it  was  Faraday's  good  fortune  to  strike  such  lodes  of 
scientific  truth  as  give  occupation  to  some  of  the  best 
intellects  of  our  age. 

The  salient  quality  of  Faraday's  scientific  character 
reveals  itself  from  beginning  to  end  of  these  volumes; 
a  union  of  ardour  and  patience — the  one  prompting 
the  attack,  the  other  holding  him  on  to  it,  till  defeat 
was  final  or  victory  assured.  Certainty  in  one  sense  or 
the  other  was  necessary  to  his  peace  of  mind.  The 
right  method  of  investigation  is  perhaps  incommuni- 
cable; it  depends  on  the  individual  rather  than  on  the 
system,  and  the  mark  is  missed  when  Faraday's  re- 
searches are  pointed  to  as  merely  illustrative  of  the 
power  of  the  inductive  philosophy.  The  brain  may  be 
filled  with  that  philosophy;  but  without  the  energy 
and  insight  which  this  man  possessed,  and  which  with 
him  were  personal  and  distinctive,  we  should  never  rise 
to  the  level  of  his  achievements.  His  power  is  that  of 
individual  genius,  rather  than  of  philosophic  method; 
the  energy  of  a  strong  soul  expressing  itself  after  its 
own  fashion,  and  acknowledging  no  mediator  between 
it  and  Nature. 

The  second  volume  of  the  '  Life  and  Letters,'  like 
the  first,  is  a  historic  treasury  as  regards  Faraday's 
work  and  character,  and  his  scientific  and  social  rela- 
tions. It  contains  letters  from  Humboldt,  Herschel, 
Hachette,  De  la  Eive,  Dumas,  Liebig,  Melloni,  Bec- 
querel,  Oersted,  Pliicker,  Du  Bois  Eeymond,  Lord 
Melbourne,  Prince  Louis  Napoleon,  and  many  other 
distinguished  men.  I  notice  with  particular  pleasure  a 
letter  from  Sir  John  Herschel,  in  reply  to  a  sealed 
packet  addressed  to  him  by  Faraday,  but  which  he  had 
permission  to  open  if  he  pleased.  The  packet  referred 
to  one  of  the  many  unfulfilled  hopes  which  spring  up 
in  the  minds  of  fertile  investigators: — 


FARADAY.  415 

'  Go  on  and  prosper,  "  from  strength  to  strength," 
like  a  victor  marching  with  assured  step  to  further 
conquests;  and  be  certain  that  no  voice  will  join  more 
heartily  in  the  peans  that  already  begin  to  rise,  and 
will  speedily  swell  into  a  shout  of  triumph,  astounding 
even  to  yourself,  than  that  of  J.  F.  W.  Herschel.' 

Faraday's  behaviour  to  Melloni  in  1835  merits  a 
word  of  notice.  The  young  man  was  a  political  exile 
in  Paris.  He  had  newly  fashioned  and  applied  the 
thermo-electric  pile,  and  had  obtained  with  it  results 
of  the  greatest  importance.  But  they  were  not  appre- 
ciated. With  the  sickness  of  disappointed  hope  Mel- 
loni waited  for  the  report  of  the  Commissioners,  ap- 
pointed by  the  Academy  of  Sciences  to  examine  the 
Primier.  At  length  he  published  his  researches  in  the 
'  Annales  de  Chimie/  They  thus  fell  into  the  hands  of 
Faraday,  who,  discerning  at  once  their  extraordinary 
merit,  obtained  for  their  author  the  Eumford  Medal  of 
the  Royal  Society.  A  sum  of  money  always  accom- 
panies this  medal;  and  the  pecuniary  help  was,  at  this 
time,  even  more  essential  than  the  mark  of  honour 
to  the  young  refugee.  Melloni's  gratitude  was  bound- 
less:— 

'Et  vous,  monsieur,'  he  writes  to  Faraday,  'qui 
appartenez  a  une  societe  a  laquelle  je  n'avais  rien  offert, 
vous  qui  me  connaissiez  a  peine  de  nomj  vous  n'avez 
pas  demande  si  j'avais  des  ennemis  faibles  ou  puissants, 
ni  calcule  quel  en  6tait  le  nombre;  mais  vous  avez 
parle  pour  1'opprime  Stranger,  pour  celui  qui  n'avait 
pas  le  moindre  droit  &  tant  de  bienveillance,  et  vos 
paroles  ont  6te"  accueillies  favorablement  par  des  col- 
legues  consciencieux!  Je  reconnais  bien  1&  des  hommes 
dignes  de  leur  noble  mission,  les  v6ritable  repr6sen- 
tants  de  la  science  d'un  pays  libre  et  g6n6reux.' 

Within  the  prescribed  limits  of  this  article  it  would 


416  FKAGMENTS    OF    SCIENCE. 

be  impossible  to  give  even  the  slenderest  summary  of 
Faraday's  correspondence,  or  to  carve  from  it  more 
than  the  merest  fragments  of  his  character.  His  let- 
ters, written  to  Lord  Melbourne  and  others  in  1836,  re- 
garding his  pension,  illustrate  his  uncompromising  in- 
dependence. The  Prime  Minister  had  offended  him, 
but  assuredly  the  apology  demanded  and  given  was 
complete.  I  think  it  certain  that,  notwithstanding  the 
very  full  account  of  this  transaction  given  by  Dr.  Bence 
Jones,  motives  and  influences  were  at  work  which  even 
now  are  not  entirely  revealed.  The  minister  was  bit- 
terly attacked,  but  he  bore  the  censure  of  the  press 
with  great  dignity.  Faraday,  while  he  disavowed  hav- 
ing either  directly  or  indirectly  furnished  the  matter 
of  those  attacks,  did  not  publicly  exonerate  the  Pre- 
mier. The  Hon.  Caroline  Fox  had  proved  herself 
Faraday's  ardent  friend,  and  it  was  she  who  had  healed 
the  breach  between  the  philosopher  and  the  minister. 
She  manifestly  thought  that  Faraday  ought  to  have 
come  forward  in  Lord  Melbourne's  defence,  and  there 
is  a  flavour  of  resentment  in  one  of  her  letters  to  him 
on  the  subject.  No  doubt  Faraday  had  good  grounds 
for  his  reticence,  but  they  are  to  me  unknown. 

In  1841  his  health  broke  down  utterly,  and  he  went 
to  Switzerland  with  his  wife  and  brother-in-law.  His 
bodily  vigour  soon  revived,  and  he  accomplished  feats 
of  walking  respectable  even  for  a  trained  mountaineer. 
The  published  extracts  from  his  Swiss  journal  contain 
many  beautiful  and  touching  allusions.  Amid  refer- 
ences to  the  tints  of  the  Jungfrau,  the  blue  rifts  of  the 
glaciers,  and  the  noble  Niesen  towering  over  the  Lake 
of  Thun,  we  come  upon  the  charming  little  scrap 
which  I  have  elsewhere  quoted:  '  Clout-nail  making 
goes  on  here  rather  considerably,  and  is  a  very  neat  and 
pretty  operation  to  observe.  I  love  a  smith's  shop  and 


FARADAY.  417 

anything  relating  to  smithery.  My  father  was  a  smith.' 
This  is  from  his  journal;  but  he  is  unconsciously 
speaking  to  somebody — perhaps  to  the  world. 

His  description  of  the  Staubbach,  Giessbach,  and 
of  the  scenic  effects  of  sky  and  mountain,  are  all  fine 
and  sympathetic.  But  amid  it  all,  and  in  reference  to 
it  all,  he  tells  his  sister  that  '  true  enjoyment  is  from 
within,  not  from  without/  In  those  days  Agassiz  was 
living  under  a  slab  of  gneiss  on  the  glacier  of  the  Aar. 
Faraday  met  Forbes  at  the  Grimsel,  and  arranged  with 
him  an  excursion  to  the  '  Hotel  des  Neuchatelois ';  but 
indisposition  put  the  project  out.  «. 

From  the  Fort  of  Ham,  in  1843,  Faraday  received 
a  letter  addressed  to  him  by  Prince  Louis  Napoleon 
Bonaparte.  He  read  this  letter  to  me  many  years  ago, 
and  the  desire,  shown  in  various  ways  by  the  French 
Emperor,  to  turn  modern  science  to  account,  has  often 
reminded  me  of  it  since.  At  the  age  of  thirty-five  the 
prisoner  of  Ham  speaks  of  *  rendering  his  captivity 
less  sad  by  studying  the  great  discoveries '  which  sci- 
ence owes  to  Faraday;  and  he  asks  a  question  which 
reveals  his  cast  of  thought  at  the  time:  'What  is  the 
most  simple  combination  to  give  to  a  voltaic  battery, 
in  order  to  produce  a  spark  capable  of  setting  fire  to 
powder  under  water  or  under  ground? '  Should  the 
necessity  arise,  the  French  Emperor  will  not  lack  at 
the  outset  the  best  appliances  of  modern  science;  while 
we,  I  fear,  shall  have  to  learn  the  magnitude  of  the 
resources  we  are  now  neglecting  amid  the  pangs  of 
actual  war.* 

One  turns  with  renewed  pleasure  to  Faraday's  let- 

*  The  'science'  has  since  been  applied,  with  astonishing 
effect,  by  those  who  had  studied  it  far  more  thoroughly  than  the 
Emperor  of  the  French.  We  also,  I  am  happy  to  think,  have 
improved  the  time  since  the  above  words  were  written  [1878]. 


418  FEAGMENTS    OF    SCIENCE. 

ters  to  his  wife,  published  in  the  second  volume.  Here 
surely  the  loving  essence  of  the  man  appears  more  dis- 
tinctly than  anywhere  else.  From  the  house  of  Dr. 
Percy,  in  Birmingham,  he  writes  thus: — 

*  Here — even  here — the  moment  I  leave  the  table, 
I  wish  I  were  with  you  IN  QUIET.    Oh,  what  happiness 
is  ours!    My  runs  into  the  world  in  this  way  only  serve 
to  make  me  esteem  that  happiness  the  more.' 
And  again: 

( We  have  been  to  a  grand  conversazione  in  the 
town-hall,  and  I  have  now  returned  to  my  room  to 
talk  with  you,  as  the  pleasantest  and  happiest  thing 
that  I  can  do.  Nothing  rests  me  so  much  as  com- 
munion with  you.  I  feel  it  even  now  as  I  write,  and 
catch  myself  saying  the  words  aloud  as  I  write  them.' 
Take  this,  moreover,  as  indicative  of  his  love  for  Na- 
ture: 

( After  writing,  I  walk  out  in  the  evening  hand  in 
hand  with  my  dear  wife  to  enjoy  the  sunset;  for  to 
me  who  love  scenery,  of  all  that  I  have  seen  or  can  see, 
there  is  none  surpasses  that  of  heaven.  A  glorious  sun- 
set brings  with  it  a  thousand  thoughts  that  delight  me.' 

Of  the  numberless  lights  thrown  upon  him  by  the 
e  Life  and  Letters,'  some  fall  upon  his  religion.  In  a 
letter  to  Lady  Lovelace,  he  describes  himself  as  belong- 
ing to  '  a  very  small  and  despised  sect  of  Christians, 
known,  if  known  at  all,  as  Sandemanians,  and  our 
hope  is  founded  on  the  faith  that  is  in  Christ.'  He 
adds: '  I  do  not  think  it  at  all  necessary  to  tie  the  study 
of  the  natural  sciences  and  religion  together,  and  in 
my  intercourse  with  my  fellow-creatures,  that  which 
is  religious,  and  that  which  is  philosophical,  have  ever 
been  two  distinct  things/  He  saw  clearly  the  danger 
of  quitting  his  moorings,  and  his  science  acted  indi- 
rectly as  the  safeguard  of  his  faith.  For  his  investiga- 


FARADAY.  419 

tions  so  filled  his  mind  as  to  leave  no  room  for  sceptical 
questionings,  thus  shielding  from  the  assaults  of  phi- 
losophy the  creed  of  his  youth.  His  religion  was  con- 
stitutional and  hereditary.  It  was  implied  in  the 
eddies  of  his  blood  and  in  the  tremors  of  his  brain; 
and,  however  its  outward  and  visible  form  might  have 
changed,  Faraday  would  still  have  possessed  its  ele- 
mental constituents — awe,  reverence,  truth,  and  love.. 

It  is  worth  enquiring  how  so  profoundly  religious  a 
mind,  and  so  great  a  teacher,  would  be  likely  to  regard 
our  present  discussions  on  the  subject  of  education. 
Faraday  would  be  a  'secularist'  were  he  now  alive. 
He  had  no  sympathy  with  those  who  contemn  knowl- 
edge unless  it  be  accompanied  by  dogma.  A  lecture 
delivered  before  the  City  Philosophical  Society  in 
1818,  when  he  was  twenty-six  years  of  age,  expresses 
the  views  regarding  education  which  he  entertained  to 
the  end  of  his  life.  '  First,  then,'  he  says,  f  all  theo- 
logical considerations  are  banished  from  the  society, 
and  of  course  from  my  remarks;  and  whatever  I  may 
say  has  no  reference  to  a  future  state,  or  to  the  means 
which  are  to  be  adopted  in  this  world  in  anticipation  of 
it.  Next,  I  have  no  intention  of  substituting  anything 
for  religion,  but  I  wish  to  take  that  part  of  human 
nature  which  is  independent  of  it.  Morality,  phi- 
losophy, commerce,  the  various  institutions  and  habits 
of  society,  are  independent  of  religion,  and  may  exist 
either  with  or  without  it.  They  are  always  the  same, 
and  can  dwell  alike  in  the  breasts  of  those  who,  from 
opinion,  are  entirely  opposed  in  the  set  of  principles 
they  include  in  the  term  religion,  or  in  those  who  have 
none. 

'To  discriminate  more  closely,  if  possible,  I  will 
observe  that  we  have  no  right  to  judge  religious  opin- 
ions; but  the  human  nature  of  this  evening  is  that 


420  FRAGMENTS    OF    SCIENCE. 

part  of  man  which  we  have  a  right  to  judge.  And  I 
think  it  will  be  found  on  examination,  that  this  hu- 
manity— as  it  may  perhaps  be  called — will  accord  with 
what  I  have  before  described  as  being  in  our  own 
hands  so  improvable  and  perfectible.' 

In  an  old  journal  I  find  the  following  remarks  on 
one  of  my  earliest  dinners  with  Faraday:  'At  two 
o'clock  he  came  down  for  me.  He,  his  niece,  and  my- 
self, formed  the  party,  "  I  never  give  dinners,"  he 
said.  "  I  don't  know  how  to  give  dinners,  and  I 
never  dine  out.  But  I  should  not  like  my  friends  to 
attribute  this  to  a  wrong  cause.  I  act  thus  for  the 
sake  of  securing  time  for  work,  and  not  through  re- 
ligious motives,  as  some  imagine."  He  said  grace.  I 
am  almost  ashamed  to  call  his  prayer  a  "  saying  of 
grace."  In  the  language  of  Scripture,  it  might  be 
described  as  the  petition  of  a  son,  into  whose  heart 
God  had  sent  the  Spirit  of  His  Son,  and  who  with 
absolute  trust  asked  a  blessing  from  his  father.  We 
dined  on  roast  beef,  Yorkshire  pudding,  and  potatoes; 
drank  sherry,  talked  of  research  and  its  requirements, 
and  of  his  habit  of  keeping  himself  free  from  the  dis- 
tractions of  society.  He  was  bright  and  joyful — boy- 
like,  in  fact,  though  he  is  now  sixty-two.  His  work 
excites  admiration,  but  contact  with  him  warms  and 
elevates  the  heart.  Here,  surely,  is  a  strong  man.  I 
love  strength;  but  let  me  not  forget  the  example  of 
its  union  with  modesty,  tenderness,  and  sweetness,  in 
the  character  of  Faraday.' 

Faraday's  progress  in  discovery,  and  the  salient 
points  of  his  character,  are  well  brought  out  by  the 
wise  choice  of  letters  and  extracts  published  in  the 
volumes  before  us.  I  will  not  call  the  labours  of  the 
biographer  final.  So  great  a  character  will  challenge 
reconstruction.  In  the  coming  time  some  sympathetic 


FARADAY.  421 

spirit,  with  the  requisite  strength,  knowledge,  and 
solvent  power,  will,  I  doubt  not,  render  these  materials 
plastic,  give  them  more  perfect  organic  form,  and  send 
through  them,  with  less  of  interruption,  the  currents 
of  Faraday's  life.  '  He  was  too  good  a  man/  writes  his 
present  biographer,  'for  me  to  estimate  rightly,  and  too 
great  a  philosopher  for  me  to  understand  thoroughly/ 
That  may  be:  but  the  reverent  affection  to  which  we 
owe  the  discovery,  selection,  and  arrangement  of  the 
materials  here  placed  before  us,  is  probably  a  surer 
guide  than  mere  literary  skill.  The  task  of  the  artist 
who  may  wish  in  future  times  to  reproduce  the  real 
though  unobtrusive  grandeur,  the  purity,  beauty,  and 
childlike  simplicity  of  him  whom  we  have  lost,  will  find 
his  chief  treasury  already  provided  for  him  by  Dr." 
Bence  Jones's  labour  of  love. 


XIX. 

THE  COPLEY  MEDALIST  OF  1870. 

PTHHIRTY  years  ago  Electro-magnetism  was  looked 
-L  to  as  a  motive  power,  which  might  possibly  com- 
pete with  steam.  In  centres  of  industry,  such  as  Man- 
chester, attempts  to  investigate  and  apply  this  power 
were  numerous.  This  is  shown  by  the  scientific  litera- 
ture of  the  time.  Among  others  Mr.  James  Prescot 
Joule,  a  resident  of  Manchester,  took  up  the  subject, 
and,  in  a  series  of  papers  published  in  Sturgeon's 
'Annals  of  Electricity'  between  1839  and  1841,  de-* 
scribed  various  attempts  at  the  construction  and  per- 
fection of  electro-magnetic  engines.  The  spirit  in 
which  Mr.  Joule  pursued  these  enquiries  is  revealed  in 
the  following  extract:  'I  am  particularly  anxious/  he 
says,  '  to  communicate  any  new  arrangement  in  order, 
if  possible,  to  forestall  the  monopolising  designs  of 
those  who  seem  to  regard  this  most  interesting  subject 
merely  in  the  light  of  pecuniary  speculation.'  He  was 
naturally  led  to  investigate  the  laws  of  electro-magnetic 
attractions,  and  in  1840  he  announced  the  important 
principle  that  the  attractive  force  exerted  by  two  elec- 
tro-magnets, or  by  an  electro-magnet  and  a  mass  of  an- 
nealed iron,  is  directly  proportional  to  the  square  of 
the  strength  of  the  magnetising  current;  while  the  at- 
traction exerted  between  an  electro-magnet  and  the 
pole  of  a  permanent  steel  magnet,  varies  simply  as  the 
422 


THE    COPLEY    MEDALIST    OF    1870.  433 

strength  of  the  current.  These  investigations  were 
conducted  independently  of,  though  a  little  subse- 
quently to,  the  celebrated  enquiries  of  Henry,  Jacobi, 
and  Lenz  and  Jacobi,  on  the  same  subject. 

On  December  17, 1840,  Mr.  Joule  communicated  to 
the  Royal  Society  a  paper  on  the  production  of  heat 
by  Voltaic  electricity.  In  it  he  announced  the  law  that 
the  calorific  effects  of  equal  quantities  of  transmitted 
electricity  are  proportional  to  the  resistance  overcome 
by  the  current,  whatever  may  be  the  length,  thickness, 
shape,  or  character  of  the  metal  which  closes  the  cir- 
cuit; and  also  proportional  to  the  square  of  the  quan- 
tity of  transmitted  electricity.  This  is  a  law  of  primary 
importance.  In  another  paper,  presented  to,  but  de- 
clined by,  the  Royal  Society,  he  confirmed  this  law  by 
new  experiments,  and  materially  extended  it.  He  also 
executed  experiments  on  the  heat  consequent  on  the 
passage  of  Voltaic  electricity  through  electrolytes,  and 
found,  in  all  cases,  that  the  heat  evolved  by  the  proper 
action  of  any  Voltaic  current  is  proportional  to  the 
square  of  the  intensity  of  that  current,  multiplied  by 
the  resistance  to  conduction  which  it  experiences. 
From  this  law  he  deduced  a  number  of  conclusions  of 
the  highest  importance  to  electro-chemistry. 

It  was  during  these  enquiries,  which  are  marked 
throughout  by  rare  sagacity  and  originality,  that  the 
great  idea  of  establishing  quantitative  relations  be- 
tween Mechanical  Energy  and  Heat  arose  and  assumed 
definite  form  in  his  mind.  In  1843  Mr.  Joule  read  be- 
fore the  meeting  of  the  British  Association  at  Cork  a 
paper  '  On  the  Calorific  Effects  of  Magneto-Electricity, 
and  on  the  Mechanical  Value  of  Heat.'  Even  at  the 
present  day  this  memoir  is  tough  reading,  and  at  the 
time  it  was  written  it  must  have  appeared  hopelessly 
entangled.  This,  I  should  think,  was  the  reason  why 


424  FRAGMENTS    OP    SCIENCE. 

Faraday  advised  Mr.  Joule  not  to  submit  the  paper  to 
the  Eoyal  Society.  But  its  drift  and  results  are  summed 
up  in  these  memorable  words  by  its  author,  written 
some  time  subsequently:  '  In  that  paper  it  was  demon- 
strated experimentally,  that  the  mechanical  power  ex- 
erted in  turning  a  magneto-electric  machine  is  con- 
verted into  the  heat  evolved  by  the  passage  of  the 
currents  of  induction  through  its  coils;  and,  on  the 
other  hand,  that  the  motive  power  of  the  electro- 
magnetic engine  is  obtained  at  the  expense  of  the  heat 
due  to  the  chemical  reaction  of  the  battery  by  which 
it  is  worked.'  *  It  is  needless  to  dwell  upon  the  weight 
and  importance  of  this  statement. 

Considering  the  imperfections  incidental  to  a  first 
determination,  it  is  not  surprising  that  the  '  mechanical 
values  of  heat/  deduced  from  the  different  series  of 
experiments  published  in  1843,  varied  widely  from 
each  other.  The  lowest  limit  was  587,  and  the  highest 
1,026  foot-pounds,  for  1°  Fahr.  of  temperature. 

One  noteworthy  result  of  his  enquiries,  which  was 
pointed  out  at  the  time  by  Mr.  Joule,  had  reference  to 
the  exceedingly  small  fraction  of  the  heat  actually 
converted  into  useful  effect  in  the  steam-engine.  The 
thoughts  of  the  celebrated  Julius  Robert  Mayer,  who 
was  then  engaged  in  Germany  upon  the  same  question, 
had  moved  independently  in  the  same  groove;  but  to 
his  labours  due  reference  will  be  made  on  a  future 
occasion.f  In  the  memoir  now  referred  to,  Mr.  Joule 
also  announced  that  he  had  proved  heat  to  be  evolved 
during  the  passage  of  water  through  narrow  tubes;  and 
he  deduced  from  these  experiments  an  equivalent  of 
770  foot-pounds,  a  figure  remarkably  near  the  one  now 
accepted.  A  detached  statement  regarding  the  origin 
and  convertibility  of  animal  heat  strikingly  illustrates 

*  Phil.  Mag.,  May,  1845.         -f  See  the  next  Fragment. 


THE    COPLEY    MEDALIST    OF    1870.  425 

the  penetration  of  Mr.  Joule,  and  his  mastery  of  prin- 
ciples, at  the  period  now  referred  to.  A  friend  had 
mentioned  to  him  nailer's  hypothesis,  that  animal  heat 
might  arise  from  the  friction  of  the  blood  in  the  veins 
and  arteries.  '  It  is  unquestionable,'  writes  Mr.  Joule, 
'  that  heat  is  produced  by  such  friction;  but  it  must  be 
understood  that  the  mechanical  force  expended  in  the 
friction  is  a  part  of  the  force  of  affinity  which  causes 
the  venous  blood  to  unite  with  oxygen,  so  that  the 
whole  heat  of  the  system  must  still  be  referred  to  the 
chemical  changes.  But  if  the  animal  were  engaged  in 
turning  a  piece  of  machinery,  or  in  ascending  a  moun- 
tain, I  apprehend  that  in  proportion  to  the  muscular 
effort  put  forth  for  the  purpose,  a  diminution  of  the 
heat  evolved  in  the  system  by  a  given  chemical  action 
would  be  experienced.'  The  italics  in  this  memorable 
passage,  written,  it  is  to  be  remembered,  in  1843,  are 
Mr.  Joule's  own. 

The  concluding  paragraph  of  this  British  Associa- 
tion paper  equally  illustrates  his  insight  and  precision, 
regarding  the  nature  of  chemical  and  latent  heat.  '  I 
had,'  he  writes,  '  endeavoured  to  prove  that  when  two 
atoms  combine  together,  the  heat  evolved  is  exactly 
that  which  would  have  been  evolved  by  the  electrical 
current  due  to  the  chemical  action  taking  place,  and  is 
therefore  proportional  to  the  intensity  of  the  chemical 
force  causing  the  atoms  to  combine.  I  now  venture  to 
state  more  explicitly,  that  it  is  not  precisely  the  attrac- 
tion of  affinity,  but  rather  the  mechanical  force  ex- 
pended by  the  atoms  in  falling  towards  one  another, 
which  determines  the  intensity  of  the  current,  and, 
consequently,  the  quantity  of  heat  evolved;  so  that  we 
have  a  simple  hypothesis  by  which  we  may  explain  why 
heat  is  evolved  so  freely  in  the  combination  of  gases, 
and  by  which  indeed  we  may  account  "  latent  heat "  as 


426  FKAGMENTS    OF    SCIENCE. 

a  mechanical  power,  prepared  for  action,  as  a  watch- 
spring  is  when  wound  up.  Suppose,  for  the  sake  of 
illustration,  that  8  Ibs.  of  oxygen  and  1  Ib.  of  hydrogen 
were  presented  to  one  another  in  the  gaseous  state,  and 
then  exploded;  the  heat  evolved  would  be  about  1° 
Fahr.  in  60,000  Ibs.  of  water,  indicating  a  mechanical 
force,  expended  in  the  combination,  equal  to  a  weight 
of  about  50,000,000  Ibs.  raised  to  the  height  of  one 
foot.  Now  if  the  oxygen  and  hydrogen  could  be  pre- 
sented to  each  other  in  a  liquid  state,  the  heat  of  com- 
bination would  be  less  than  before,  because  the  atoms 
in  combining  would  fall  through  less  space.'  No 
words  of  mine  are  needed  to  point  out  the  com- 
manding grasp  of  molecular  physics,  in  their  relation 
to  the  mechanical  theory  of  heat,  implied  by  this  state- 
ment. 

Perfectly  assured  of  the  importance  of  the  principle 
which  his  experiments  aimed  at  establishing,  Mr.  Joule 
did  not  rest  content  with  results  presenting  such  dis- 
crepancies as  those  above  referred  to.  He  resorted  in 
1844  to  entirely  new  methods,  and  made  elaborate  ex- 
periments on  the  thermal  changes  produced  in  air  dur- 
ing its  expansion:  firstly,  against  a  pressure,  and  there- 
fore performing  work;  secondly,  against  no  pressure, 
and  therefore  performing  no  work.  He  thus  estab- 
lished anew  the  relation  between  the  heat  consumed  and 
the  work  done.  From  five  different  series  of  experi- 
ments he  deduced  five  different  mechanical  equivalents; 
the  agreement  between  them  being  far  greater  than 
that  attained  in  his  first  experiments.  The  mean  of 
them  was  802  foot-pounds.  From  experiments  with 
water  agitated  by  a  paddle-wheel,  he  deduced,  in 
1845,  an  equivalent  of  890  foot-pounds.  In  1847  he 
again  operated  upon  water  and  sperm-oil,  agitated 
them  by  a  paddle-wheel,  determined  their  elevation  of 


THE    COPLEY    MEDALIST    OF    1870.  427 

temperature,  and  the  mechanical  power  which  pro- 
duced it.  From  the  one  he  derived  an  equivalent  of 
781.5  foot-pounds;  from  the  other  an  equivalent  of 
782.1  foot-pounds.  The  mean  of  these  two  very  close 
determinations  is  781.8  foot-pounds. 

By  this  time  the  labours  of  the  previous  ten  years 
had  made  Mr.  Joule  completely  master  of  the  condi- 
tions essential  to  accuracy  and  success.  Bringing  his 
ripened  experience  to  bear  upon  the  subject,  he  exe- 
cuted in  1849  a  series  of  40  experiments  on  the  friction 
of  water,  50  experiments  on  the  friction  of  mercury, 
and  20  experiments  on  the  friction  of  plates  of  cast- 
iron.  He  deduced  from  these  experiments  our  present 
mechanical  equivalent  of  heat,  just  recognised  all  over 
the  world  as  '  Joule's  equivalent/ 

There  are  labours  so  great  and  so  pregnant  in  con- 
sequences, that  they  are  most  highly  praised  when  they 
are  most  simply  stated.  Such  are  the  labours  of  Mr. 
Joule.  They  constitute  the  experimental  foundation 
of  a  principle  of  incalculable  moment,  not  only  to  the 
practice,  but  still  more  to  the  philosophy  of  Science. 
Since  the  days  of  Newton,  nothing  more  important 
than  the  theory,  of  which  Mr.  Joule  is  the  experi- 
mental demonstrator,  has  been  enunciated. 

I  have  omitted  all  reference  to  the  numerous  minor 
papers  with  which  Mr.  Joule  has  enriched  scientific 
literature.  Nor  have  I  alluded  to  the  important  in- 
vestigations which  he  has  conducted  jointly  with  Sir 
William  Thomson.  But  sufficient,  I  think,  has  been 
here  said  to  show  that,  in  conferring  upon  Mr.  Joule 
the  highest  honour  of  the  Royal  Society,  the  Council 
paid  to  genius  not  only  a  well-won  tribute,  but  one 
which  had  been  fairly  earned  twenty  years  previously.* 

*  Lord  Beaconsfield  has  recently  honoured  himself  and  Eng- 
land by  bestowing  an  annual  pension  of  200/.  on  Dr.  Joule. 


XX. 

TEE  COPLEY  MEDALIST  OF  1871. 

DR.  JULIUS  ROBERT  MAYER  was  educated  for 
the  medical  profession.  In  the  summer  of 
1840,  as  he  himself  informs  us,  he  was  at  Java,  and 
there  observed  that  the  venous  blood  of  some  of  his 
patients  had  a  singularly  bright  red  colour.  The  ob- 
servation riveted  his  attention;  he  reasoned  upon  it, 
and  came  to  the  conclusion  that  the  brightness  of  the 
colour  was  due  to  the  fact  that  a  less  amount  of  oxida- 
tion sufficed  to  keep  up  the  temperature  of  the  body  in 
a  hot  climate  than  in  a  cold  one.  The  darkness  of  the 
venous  blood  he  regarded  as  the  visible  sign  of  the 
energy  of  the  oxidation. 

It  would  be  trivial  to  remark  that  accidents  such  as 
this,  appealing  to  minds  prepared  for  them,  have  often 
led  to  great  discoveries.  Mayer's  attention  was  thereby 
drawn  to  the  whole  question  of  animal  heat.  Lavoisier 
had  ascribed  this  heat  to  the  oxidation  of  the  food. 
'  One  great  principle,'  says  Mayer, '  of  the  physiological 
theory  of  combustion,  is  that  under  all  circumstances 
the  same  amount  of  fuel  yields,  by  its  perfect  combus- 
tion, the  same  amount  of  heat;  that  this  law  holds 
good  even  for  vital  processes;  and  that  hence  the  living 
body,  notwithstanding  all  its  enigmas  and  wonders,  is 
incompetent  to  generate  heat  out  of  nothing.' 

But  beyond  the  power  of  generating  internal  heat, 
428 


THE    COPLEY    MEDALIST    OF    1871.  439 

the  animal  organism  can  also  generate  heat  outside  of 
itself.  A  blacksmith,  for  example,  by  hammering  can 
heat  a  nail,  and  a  savage  by  friction  can  warm  wood  to 
its  point  of  ignition.  Now,  unless  we  give  up  the 
physiological  axiom  that  the  living  body  cannot  create 
heat  out  of  nothing,  '  we  are  driven/  says  Mayer,  '  to 
the  conclusion  that  it  is  the  total  heat  generated  within 
and  without  that  is  to  be  regarded  as  the  true  calorific 
effect  of  the  matter  oxidised  in  the  body.' 

From  this,  again,  he  inferred  that  the  heat  gener- 
ated externally  must  stand  in  a  fixed  relation  to  the 
work  expended  in  its  production.  For,  supposing  the 
organic  processes  to  remain  the  same;  if  it  were  possi- 
ble, by  the  mere  alteration  of  the  apparatus,  to  generate 
different  amounts  of  heat  by  the  same  amount  of  work, 
•it  would  follow  that  the  oxidation  of  the  same  amount 
of  material  would  sometimes  yield  a  less,  sometimes  a 
greater,  quantity  of  heat.  '  Hence/  says  Mayer,  '  that 
a  fixed  relation  subsists  between  heat  and  work,  is  a 
postulate  of  the  physiological  theory  of  combustion.' 

This  is  the  simple  and  natural  account,  given  sub- 
sequently by  Mayer  himself,  of  the  course  of  thought 
started  by  his  observation  in  Java.  But  the  conviction 
once  formed,  that  an  unalterable  relation  subsists  be- 
tween work  and  heat,  it  was  inevitable  that  Mayer 
should  seek  to  express  it  numerically.  It  was  also  in- 
evitable that  a  mind  like  his,  having  raised  itself  to 
clearness  on  this  important  point,  should  push  forward 
to  consider  the  relationship  of  natural  forces  generally. 
At  the  beginning  of  1842  his  work  had  made  consider- 
able progress;  but  he  had  become  physician  to  the 
town  of  Heilbronn,  and  the  duties  of  his  profession 
limited  the  time  which  he  could  devote  to  purely  sci- 
entific enquiry.  He  thought  it  wise,  therefore,  to  se- 
cure himself  against  accident,  and  in  the  spring  of 


430  FEAGMENTS    OF    SCIENCE. 

1842  wrote  to  Liebig,  asking  him  to  publish  in  his 

*  Annalen '  a  brief  preliminary  notice  of  the  work  then 
accomplished.     Liebig  did  so,  and  Dr.  Mayer's  first 
paper  is  contained  in  the  May  number  of  the  *  An- 
nalen '  for  1842. 

Mayer  had  reached  his  conclusions  by  reflecting  on 
the  complex  processes  of  the  living  body;  but  his  first 
step  in  public  was  to  state  definitely  the  physical  prin- 
ciples on  which  his  physiological  deductions  were  to 
rest.  He  begins,  therefore,  with  the  forces  of  inor- 
ganic nature.  He  finds  in  the  universe  two  systems  of 
causes  which  are  not  mutually  convertible; — the  dif- 
ferent kinds  of  matter  and  the  different  forms  of  force. 
The  first  quality  of  both  he  affirms  to  be  indestructi- 
bility. A  force  cannot  become  nothing,  nor  can  it  arise 
from  nothing.  Forces  are  convertible  but  not  de- 
structible. In  the  terminology  of  his  time,  he  then 
gives  clear  expression  to  the  ideas  of  potential  and 
dynamic  energy,  illustrating  his  point  by  a  weight 
resting  upon  the  earth,  suspended  at  a  height  above 
the  earth,  and  actually  falling  to  the  earth.  He  next 
fixes  his  attention  on  cases  where  motion  is  apparently 
destroyed,  without  producing  other  motion;  on  the 
shock  of  inelastic  bodies,  for  example.  Under  what 
form  does  the  vanished  motion  maintain  itself?  Ex- 
periment alone,  says  Mayer,  can  help  us  here.  He 
warms  water  by  stirring  it;  he  refers  to  the  force  ex- 
pended in  overcoming  friction.  Motion  in  both  cases 
disappears;  but  heat  is  generated,  and  the  quantity 
generated  is  the  equivalent  of  the  motion  destroyed. 

*  Our   locomotives/   he    observes   with    extraordinary 
sagacity,  *  may  be  compared  to  distilling  apparatus: 
the  heat  beneath  the  boiler  passes  into  the  motion  of 
the  train,  and  is  again  deposited  as  heat  in  the  axles 
and  wheels.' 


THE    COPLEY   MEDALIST    OF    1871.  431 

A  numerical  solution  of  the  relation  between  heat 
and  work  was  what  Mayer  aimed  at,  and  towards  the 
end  of  his  first  paper  he  makes  the  attempt.  It  was 
known  that  a  definite  amount  of  air,  in  rising  one  de- 
gree in  temperature,  can  take  up  two  different  amounts 
of  heat.  If  its  volume  be  kept  constant,  it  takes  up 
one  amount;  if  its  pressure  be  kept  constant  it  takes 
up  a  different  amount.  These  two  amounts  are  called 
the  specific  heat  under  constant  volume  and  under 
constant  pressure.  The  ratio  of  the  first  to  the  second 
is  as  1  :  1.421.  No  man,  to  my  knowledge,  prior  to 
Dr.  Mayer,  penetrated  the  significance  of  these  two 
numbers.  He  first  saw  that  the  excess  0.421  was  not, 
as  then  universally  supposed,  heat  actually  lodged  in 
the  gas,  but  heat  which  had  been  actually  consumed 
by  the  gas  in  expanding  against  pressure.  The  amount 
of  work  here  performed  was  accurately  known,  the 
amount  of  heat  consumed  was  also  accurately  known, 
and  from  these  data  Mayer  determined  the  mechanical 
equivalent  of  heat.  Even  in  this  first  paper  he  is  able 
to  direct  attention  to  the  enormous  discrepancy  be- 
tween the  theoretic  power  of  the  fuel  consumed  in 
steam-engines,  and  their  useful  effect. 

Though  this  paper  contains  but  the  germ  of  his 
further  labours,  I  think  it  may  be  safely  assumed  that, 
as  regards  the  mechanical  theory  of  heat,  this  obscure 
Heilbronn  physician,  in  the  year  1842,  was  in  advance 
of  all  the  scientific  men  of  the  time. 

Having,  by  the  publication  of  this  paper,  secured 
himself  against  what  he  calls  '  Eventualitaten/  he  de- 
voted every  hour  of  his  spare  time  to  his  studies,  and  in 
1845  published  a  memoir  which  far  transcends  his  first 
one  in  weight  and  fulness,  and,  indeed,  marks  an  epoch 
in  the  history  of  science.  The  title  of  Mayer's  first 
paper  was,  *  Remarks  on  the  Forces  of  Inorganic  Na- 


432  FEAGMENTS    OF    SCIENCE. 

ture.'  The  title  of  his  second  great  essay  was, '  Organic 
Motion  in  its  Connection  with  Nutrition/  In  it  he 
expands  and  illustrates  the  physical  principles  laid 
down  in  his  first  brief  paper.  He  goes  fully  through 
the  calculation  of  the  mechanical  equivalent  of  heat. 
He  calculates  the  performances  of  steam-engines,  and 
finds  that  100  Ibs.  of  coal,  in  a  good  working  engine, 
produce  only  the  same  amount  of  heat  as  95  Ibs.  in  an 
unworking  one;  the  5  missing  Ibs.  having  been  con- 
verted into  work.  He  determines  the  useful  effect  of 
gunpowder,  and  finds  nine  per  cent,  of  the  force  of  the 
consumed  charcoal  invested  on  the  moving  ball.  He 
records  observations  on  the  heat  generated  in  water 
agitated  by  the  pulping-engine  of  a  paper  manufactory, 
and  calculates  the  equivalent  of  that  heat  in  horse- 
power. He  compares  chemical  combination  with  me- 
chanical combination — the  union  of  atoms  with  the 
union  of  falling  bodies  with  the  earth.  He  calculates 
the  velocity  with  which  a  body  starting  at  an  infinite 
distance  would  strike  the  earth's  surface,  and  finds  that 
the  heat  generated  by  its  collision  would  raise  an  equal 
weight  of  water  17,356°  C.  in  temperature.  He  then 
determines  the  thermal  effect  which  would  be  produced 
by  the  earth  itself  falling  into  the  sun.  So  that  here, 
in  1845,  we  have  the  germ  of  that  meteoric  theory  of 
the  sun's  heat  which  Mayer  developed  with  such  ex- 
traordinary ability  three  years  afterwards.  He  also 
points  to  the  almost  exclusive  efficacy  of  the  sun's  heat 
in  producing  mechanical  motions  upon  the  earth,  wind- 
ing up  with  the  profound  remark,  that  the  heat  devel- 
oped by  friction  in  the  wheels  of  our  wind  and  water 
mills  comes  from  the  sun  in  the  form  of  vibratory  mo- 
tion; while  the  heat  produced  by  mills  driven  by  tidal 
action  is  generated  at  the  expense  of  the  earth's  axial 
rotation. 


THE    COPLEY    MEDALIST    OF    1871.  433 

Having  thus,  with  firm  step,  passed  through  the 
powers  of  inorganic  nature,  his  next  object  is  to  bring 
his  principles  to  bear  upon  the  phenomena  of  vegetable 
and  animal  life.  Wood  and  coal  can  burn;  whence 
come  their  heat,  and  the  work  producible  by  that  heat? 
From  the  immeasurable  reservoir  of  the  sun.  Nature 
has  proposed  to  herself  the  task  of  storing  up  the  light 
which  streams  earthward  from  the  sun,  and  of  casting 
into  a  permanent  form  the  most  fugitive  of  all  powers. 
To  this  end  she  has  overspread  the  earth  with  organ- 
isms which,  while  living,  take  in  the  solar  light,  and 
by  its  consumption  generate  forces  of  another  kind. 
These  organisms  are  plants.  The  vegetable  world,  in- 
deed, constitutes  the  instrument  whereby  the  wave- 
motion  of  the  sun  is  changed  into  the  rigid  form  of 
chemical  tension,  and  thus  prepared  for  future  use. 
With  this  prevision,  as  shall  subsequently  be  shown, 
the  existence  of  the  human  race  itself  is  inseparably 
connected.  It  is  to  be  observed  that  Mayer's  utterances 
are  far  from  being  anticipated  by  vague  statements 
regarding  the  ' stimulus'  of  light,  or  regarding  coal 
as  *  bottled  sunlight/  He  first  saw  the  full  meaning  of 
De  Saussure's  observation  as  to  the  reducing  power  of 
the  solar  rays,  and  gave  that  observation  its  proper 
place  in  the  doctrine  of  conservation.  In  the  leaves  of 
a  tree,  the  carbon  and  oxygen  of  carbonic  acid,  and  the 
hydrogen  and  oxygen  of  water,  are  forced  asunder  at 
the  expense  of  the  sun,  and  the  amount  of  power  thus 
sacrificed  is  accurately  restored  by  the  combustion  of 
the  tree.  The  heat  and  work  potential  in  our  coal 
strata  are  so  much  strength  withdrawn  from  the  sun  of 
former  ages.  Mayer  lays  the  axe  to  the  root  of  the 
notions  regarding  ' vital  force*  which  were  prevalent 
when  he  wrote.  With  the  plain  fact  before  us  that  in 
the  absence  of  the  solar  rays  plants  cannot  perform  the 


434  FRAGMENTS    OF    SCIENCE. 

work  of  reduction,  or  generate  chemical  tensions,  it 
is,  he  contends,  incredible  that  these  tensions  should 
be  caused  by  the  mystic  play  of  the  vital  force.  Such 
an  hypothesis  would  cut  off  all  investigation;  it  would 
land  us  in  a  chaos  of  unbridled  phantasy.  ( I  count/ 
he  says,  '  therefore,  upon  your  agreement  with  me 
when  I  state,  as  an  axiomatic  truth,  that  during  vital 
processes  the  conversion  only,  and  never  the  creation 
of  matter  or  force  occurs/ 

Having  cleared  his  way  through  the  vegetable 
world,  as  he  had  previously  done  through  inorganic 
nature,  Mayer  passes  on  to  the  other  organic  kingdom. 
The  physical  forces  collected  by  plants  become  the 
property  of  animals.  Animals  consume  vegetables,  and 
cause  them  to  reunite  with  the  atmospheric  oxygen. 
Animal  heat  is  thus  produced;  and  not  only  animal 
heat,  but  animal  motion.  There  is  no  indistinctness 
about  Mayer  here;  he  grasps  his  subject  in  all  its  de- 
tails, and  reduces  to  figures  the  concomitants  of  mus- 
cular action.  A  bowler  who  imparts  to  an  8-lb.  ball  a 
velocity  of  30  feet,  consumes  in  the  act  ^  of  a  grain 
of  carbon.  A  man  weighing  150  Ibs.,  who  lifts  his 
own  body  to  a  height  of  8  feet,  consumes  in  the  act  1 
grain  of  carbon.  In  climbing  a  mountain  10,000  feet 
high,  the  consumption  of  the  same  man  would  be  2  oz. 
4  drs.  50  grs.  of  carbon.  Boussingault  had  determined 
experimentally  the  addition  to  be  made  to  the  food  of 
horses  when  actively  working,  and  Liebig  had  deter- 
mined the  addition  to  be  made  to  the  food  of  men. 
Employing  the  mechanical  equivalent  of  heat,  which 
he  had  previously  calculated,  Mayer  proves  the  addi- 
tional food  to  be  amply  sufficient  to  cover  the  increased 
oxidation. 

But  he  does  not  content  himself  with  showing,  in  a 
general  way,  that  the  human  body  burns  according  to 


THE    COPLEY    MEDALIST   OF    1871.  435 

definite  laws,  when  it  performs  mechanical  work.  He 
seeks  to  determine  the  particular  portion  of  the  body 
consumed,  and  in  doing  so  executes  some  noteworthy 
calculations.  The  muscles  of  a  labourer  150  Ibs.  in 
weight  weigh  64  Ibs.;  but  when  perfectly  desiccated 
they  fall  to  15  Ibs.  Were  the  oxidation  corresponding 
to  that  labourer's  work  exerted  on  the  muscles  alone, 
they  would  be  utterly  consumed  in  80  days.  The  heart 
furnishes  a  still  more  striking  example.  Were  the 
oxidation  necessary  to  sustain  the  heart's  action  exerted 
upon  its  own  tissue,  it  would  be  utterly  consumed  in  8 
days.  And  if  we  confine  our  attention  to  the  two 
ventricles,  their  action  would  be  sufficient  to  consume 
the  associated  muscular  tissue  in  3£  days.  Here,  in  his 
own  words,  emphasised  in  his  own  way,  is  Mayer's 
pregnant  conclusion  from  these  calculations:  *  The 
muscle  is  only  the  apparatus  by  means  of  which  the 
conversion  of  the  force  is  effected;  but  it  is  not  the 
substance  consumed  in  the  production  of  the  mechanical 
effect.'  He  calls  the  blood  'the  oil  of  the  lamp  of 
life; '  it  is  the  slow-burning  fluid  whose  chemical  force, 
in  the  furnace  of  the  capillaries,  is  sacrificed  to  produce 
animal  motion.  This  was  Mayer's  conclusion  twenty- 
six  years  ago.  It  was  in  complete  opposition  to  the 
scientific  conclusions  of  his  time;  but  eminent  investi- 
gators have  since  amply  verified  it. 

Thus,  in  baldest  outline,  I  have  sought  to  give  some 
notion  of  the  first  half  of  this  marvellous  essay.  The 
second  half  is  so  exclusively  physiological  that  I  do 
not  wish  to  meddle  with  it.  I  will  only  add  the  illus- 
tration employed  by  Mayer  to  explain  the  action  of 
the  nerves  upon  the  muscles.  As  an  engineer,  by  the 
motion  of  his  finger  in  opening  a  valve  or  loosing  a 
detent,  can  liberate  an  amount  of  mechanical  motion 
almost  infinite  compared  with  its  exciting  cause,  so 


436  FRAGMENTS    OF    SCIENCE. 

the  nerves,  acting  upon  the  muscles,  can  unlock  an 
amount  of  activity,  wholly  out  of  proportion  to  the 
work  done  by  the  nerves  themselves. 

As  regards  these  Questions  of  weightiest  import  to 
the  science  of  physiology,  Dr.  Mayer,  in  1845,  was  as- 
suredly far  in  advance  of  all  living  men. 

Mayer  grasped  the  mechanical  theory  of  heat  with 
commanding  power,  illustrating  it  and  applying  it  in 
the  most  diverse  domains.  He  began,  as  we  have  seen, 
with  physical  principles;  he  determined  the  numerical 
relation  between  heat  and  work;  he  revealed  the  source 
of  the  energies  of  the  vegetable  world,  and  showed  the 
relationship  of  the  heat  of  our  fires  to  solar  heat.  He 
followed  the  energies  which  were  potential  in  the  vege- 
table, up  to  their  local  exhaustion  in  the  animal.  But 
in  1845  a  new  thought  was  forced  upon  him  by  his 
calculations.  He  then,  for  the  first  time,  drew  atten- 
tion to  the  astounding  amount  of  heat  generated  by 
gravity  where  the  force  has  sufficient  distance  to  act 
through.  He  proved,  as  I  have  before  stated,  the  heat 
of  collision  of  a  body  falling  from  an  infinite  distance 
to  the  earth,  to  be  sufficient  to  raise  the  temperature 
of  a  quantity  of  water,  equal  to  the  falling  body  in 
weight,  17,356°  C.  He  also  found,  in  1845,  that  the 
gravitating  force  between  the  earth  and  sun  was  com' 
petent  to  generate  an  amount  of  heat  equal  to  that  ob- 
tainable from  the  combustion  of  6,000  times  the  weight 
of  the  earth  of  solid  coal.  With  the  quickness  of 
genius  he  saw  that  we  had  here  a  power  sufficient  to 
produce  the  enormous  temperature  of  the  sun,  and 
also  to  account  for  the  primal  molten  condition  of  our 
own  planet.  Mayer  shows  the  utter  inadequacy  of 
chemical  forces,  as  we  know  them,  to  produce  or  main- 
tain the  solar  temperature.  He  shows  that  were  the 
sun  a  lump  of  coal  it  would  be  utterly  consumed  in 


THE    COPLEY    MEDALIST    OF    1871.  437 

5,000  years.  He  shows  the  difficulties  attending  the 
assumption  that  the  sun  is  a  cooling  body;  for,  sup- 
posing it  to  possess  even  the  high  specific  heat  of  water, 
its  temperature  would  fall  15,000°  in  5,000  years.  He 
finally  concludes  that  the  light  and  heat  of  the  sun  are 
maintained  by  the  constant  impact  of  meteoric  matter. 
I  never  ventured  an  opinion  as  to  the  truth  of  this 
theory;  that  is  a  question  which  may  still  have  to  be 
fought  out.  But  I  refer  to  it  as  an  illustration  of  the 
force  of  genius  with  which  Mayer  followed  the  me- 
chanical theory  of  heat  through  all  its  applications. 
Whether  the  meteoric  theory  be  a  matter  of  fact  or  not, 
with  him  abides  the  honour  of  proving  to  demonstra- 
tion that  the  light  and  heat  of  suns  and  stars  may  be 
originated  and  maintained  by  the  collisions  of  cold 
planetary  matter. 

It  is  the  man  who  with  the  scantiest  data  could 
accomplish  all  this  in  six  short  years,  and  in  the  hours 
snatched  from  the  duties  of  an  arduous  profession, 
that  the  Royal  Society,  in  1871,  crowned  with  its  high- 
est honour. 

Comparing  this  brief  history  with  that  of  the  Cop- 
ley Medalist  of  1870,  the  differentiaffng  influence  of 
*  environment,'  on  two  minds  of  similar  natural  cast 
and  endowment,  comes  out  in  an  instructive  manner. 
Withdrawn  from  mechanical  appliances,  Mayer  fell 
back  upon  reflection,  selecting  with  marvellous  sagac- 
ity, from  existing  physical  data,  the  single  result  on 
which  could  be  founded  a  calculation  of  the  mechan- 
ical equivalent  of  heat.  In  the  midst  of  mechanical 
appliances,  Joule  resorted  to  experiment,  and  laid  the 
broad  and  firm  foundation  which  has  secured  for  the 
mechanical  theory  the  acceptance  it  now  enjoys.  A 
great  portion  of  Joule's  time  was  occupied  in  actual 
manipulation;  freed  from  this,  Mayer  had  time  to  fol- 


438  FRAGMENTS    OF    SCIENCE. 

low  the  theory  into  its  most  abstruse  and  impressive 
applications.  With  their  places  reversed,  however, 
Joule  might  have  become  Mayer,  and  Mayer  might 
have  become  Joule. 

It  does  not  lie  within  the  scope  of  these  brief  arti- 
cles to  enter  upon  the  developments  of  the  Dynamical 
Theory  accomplished  since  Joule  and  Mayer  executed 
their  memorable  labours. 


XXI. 

DEATH  BY  LIGHTNING. 

1  )EOPLE  in  general  imagine,  when  they  think  at 
JL  all  about  the  matter,  that  an  impression  upon 
the  nerves — a  blow,  for  example,  or  the  prick  of  a  pin 
— is  felt  at  the  moment  it  is  inflicted.  But  this  is  not 
the  case.  The  seat  of  sensation  being  the  brain,  to  it 
the  intelligence  of  any  impression  made  upon  the  nerves 
has  to  be  transmitted  before  this  impression  can  be- 
come manifest  as  consciousness.  The  transmission, 
moreover,  requires  time,  and  the  consequence  is,  that 
a  wound  inflicted  on  a  portion  of  the  body  distant  from 
the  brain  is  more  tardily  appreciated  than  one  inflicted 
adjacent  to  the  brain.  By  an  extremely  ingenious  ex- 
perimental arrangement,  Helmholtz  has  determined 
the  velocity  of  this  nervous  transmission,  and  finds  it 
to  be  about  eighty  feet  a  second,  or  less  than  one-thir- 
teenth of  the  velocity  of  sound  in  air.  If  therefore, 
a  whale  forty  feet  long  were  wounded  in  the  tail,  it 
would  not  be  conscious  of  the  injury  till  half  a  second 
after  the  wound  had  been  inflicted.*  But  this  is  not 
the  only  ingredient  in  the  delay.  There  can  scarcely 
be  a  doubt  that  to  every  act  of  consciousness  belongs 
a  determinate  molecular  arrangement  of  the  brain— 

*  A  most  admirable  lecture  on  the  velocity  of  nervous  trans- 
mission has  been  published  by  Dr.  Du  Bois  Reymond  in  the  '  Pro- 
ceedings of  the  Royal  Institution '  for  1866,  vol.  iv.  p.  575. 
29  439 


440  FRAGMENTS    OF    SCIENCE. 

that  every  thought  or  feeling  has  its  physical  correla- 
tive in  that  organ;  and  nothing  can  be  more  certain 
than  that  every  physical  change,  whether  molecular 
or  mechanical,  requires  time  for  its  accomplishment. 
So  that,  besides  the  interval  of  transmission,  a  still  fur- 
ther time  is  necessary  for  the  brain  to  put  itself  in 
order — for  its  molecules  to  take  up  the  motions  or  posi- 
tions necessary  to  the  completion  of  consciousness. 
Helmholtz  considers  that  one-tenth  of  a  second  is  de- 
manded for  this  purpose.  Thus,  in  the  case  of  the 
whale  above  supposed,  we  have  first  half  a  second  con- 
sumed in  the  transmission  of  the  intelligence  through 
the  sensor  nerves  to  the  head,  one-tenth  of  a  second 
consumed  by  the  brain  in  completing  the  arrangements 
necessary  to  consciousness,  and,  if  the  velocity  of  trans- 
mission through  the  motor  be  the  same  as  that  through 
the  sensor  nerves,  half  a  second  in  sending  a  command 
to  the  tail  to  defend  itself.  Thus  one  second  and  a 
tenth  would  elapse  before  an  impression  made  upon  its 
caudal  nerves  could  be  responded  to  by  a  whale  forty 
feet  long. 

Now,  it  is  quite  conceivable  that  an  injury  might 
be  inflicted  so  rapidly  that  within  the  time  required  by 
the  brain  to  complete  the  arrangements  necessary  to 
consciousness,  its  power  of  arrangement  might  be  de- 
stroyed. In  such  a  case,  though  the  injury  might  be 
of  a  nature  to  cause  death,  this  would  occur  without 
pain.  Death  in  this  case  would  be  simply  the  sudden 
negation  of  life,  without  any  intervention  of  conscious- 
ness whatever. 

The  time  required  for  a  rifle-bullet  to  pass  clean 
through  a  man's  head  may  be  roughly  estimated  at  a 
thousandth  of  a  second.  Here,  therefore,  we  should 
have  no  room  for  sensation,  and  death  would  be  pain- 
less. But  there  are  other  actions  which  far  transcend 


DEATH    BY   LIGHTNING.  441 

in  rapidity  that  of  the  rifle-bullet.  A  flash  of  light- 
ning cleaves  a  cloud,  appearing  and  disappearing  in 
less  than  a  hundred-thousandth  of  a  second,  and  the 
velocity  of  electricity  is  such  as  would  carry  it  in  a 
single  second  over  a  distance  almost  equal  to  that  which 
separates  the  earth  and  moon.  It  is  well  known  that  a 
luminous  impression  once  made  upon  the  retina  en- 
dures for  about  one-sixth  of  a  second,  and  that  this  is 
the  reason  why  we  see  a  continuous  band  of  light  when 
a  glowing  coal  is  caused  to  pass  rapidly  through  the 
air.  A  body  illuminated  by  an  instantaneous  flash 
continues  to  be  seen  for  the  sixth  of  a  second  after  the 
flash  has  become  extinct;  and  if  the  body  thus  illu- 
minated be  in  motion,  it  appears  at  rest  at  the  place 
where  the  flash  falls  upon  it.  When  a  colour-top  with 
differently-coloured  sectors  is  caused  to  spin  rapidly 
the  colours  blend  together.  Such  a  top,  rotating  in  a 
dark  room  and  illuminated  by  an  electric  spark,  ap- 
pears motionless,  each  distinct  colour  being  clearly 
seen.  Professor  Dove  has  found  that  a  flash  of  light- 
ning produces  the  same  effect.  During  a  thunderstorm 
he  put  a  colour-top  in  exceedingly  rapid  motion,  and 
found  that  every  flash  revealed  the  top  as  a  motionless 
object  with  its  colours  distinct.  If  illuminated  solely 
by  a  flash  of  lightning,  the  motion  of  all  bodies  on  the 
earth's  surface  would,  as  Dove  has  remarked,  appear 
suspended.  A  cannon-ball,  for  example,  would  have 
its  flight  apparently  arrested,  and  would  seem  to  hang 
motionless  in  space  as  long  as  the  luminous  impression 
which  revealed  the  ball  remained  upon  the  eye. 

If,  then,  a  rifle-bullet  move  with  sufficient  rapidity 
to  destroy  life  without  the  interposition  of  sensation, 
much  more  is  a  flash  of  lightning  competent  to  pro- 
duce this  effect.  Accordingly,  we  have  well-authen- 
ticated cases  of  people  being  struck  senseless  by  light- 


442  FRAGMENTS    OF    SCIENCE. 

ning  who,  on  recovery,  had  no  memory  of  pain.  The 
following  circumstantial  case  is  described  by  Hem- 
mer: — 

On  June  30,  1788,  a  soldier  in  the  neighbourhood 
of  Mannheim,  being  overtaken  by  rain,  placed  himself 
under  a  tree,  beneath  which  a  woman  had  previously 
taken  shelter.  He  looked  upwards  to  see  whether  the 
branches  were  thick  enough  to  afford  the  required  pro- 
tection, and,  in  doing  so,  was  struck  by  lightning,  and 
fell  senseless  to  the  earth.  The  woman  at  his  side  ex- 
perienced the  shock  in  her  foot,  but  was  not  struck 
down.  Some  hours  afterwards  the  man  revived,  but 
remembered  nothing  about  what  had  occurred,  save 
the  fact  of  his  looking  up  at  the  branches.  This  was 
his  last  act  of  consciousness,  and  he  passed  from  the 
conscious  to  the  unconscious  condition  without  pain. 
The  visible  marks  of  a  lightning  stroke  are  usually 
insignificant:  the  hair  is  sometimes  burnt;  slight 
wounds  are  observed;  while,  in  some  instances,  a  red 
streak  marks  the  track  of  the  discharge  over  the  skin. 

Under  ordinary  circumstances,  the  discharge  from 
a  small  Leyden  jar  is  exceedingly  unpleasant  to  me. 
Some  time  ago  I  happened  to  stand  in  the  presence  of 
a  numerous  audience,  with  a  battery  of  fifteen  large 
Leyden  jars  charged  beside  me.  Through  some  awk- 
wardness on  my  part,  I  touched  a^wire  leading  from 
the  battery,  and  the  discharge  went  through  my  body. 
Life  was  absolutely  blotted  out  for  a  very  sensible 
interval,  without  a  trace  of  pain.  In  a  second  or  so 
consciousness  returned;  I  vaguely  discerned  the  audi- 
ence and  apparatus,  and,  by  the  help  of  these  external 
appearances,  immediately  concluded  that  I  had  re- 
ceived the  battery  discharge.  The  intellectual  con- 
sciousness of  my  position  was  restored  with  exceeding 
rapidity,  but  not  so  the  optical  consciousness.  To  pre- 


DEATH   BY    LIGHTNING.  443 

vent  the  audience  from  being  alarmed,  I  observed  that 
it  had  often  been  my  desire  to  receive  accidentally  such 
a  shock,  and  that  my  wish  had  at  length  been  fulfilled. 
But,  while  making  this  remark,  the  appearance  which 
my  body  presented  to  my  eyes  was  that  of  a  number 
of  separate  pieces.  The  arms,  for  example,  were  de- 
tached from  the  trunk,  and  seemed  suspended  in  the 
air.  In  fact,  memory  and  the  power  of  reasoning 
appeared  to  be  complete  long  before  the  optic  nerve 
was  restored  to  healthy  action.  But  what  I  wish  chiefly 
to  dwell  upon  here  is,  the  absolute  painlessness  of  the 
shock;  and  there  cannot,  I  think,  be  a  doubt  that,  to  a 
person  struck  dead  by  lightning,  the  passage  from  life 
to  death  occurs  without  consciousness  being  in  the 
least  degree  implicated.  It  is  an  abrupt  stoppage  of 
sensation,  unaccompanied  by  a  pang. 


XXII. 

«• 

SCIENCE  AND  THE  'SPIRITS: 

1 1 1HEIK  refusal  to  investigate  '  spiritual  phenom- 
-L  ena '  is  often  urged  as  a  reproach  against  scien- 
tific men.  I  here  propose  to  give  a  sketch  of  an  at- 
tempt to  apply  to  the  '  phenomena '  those  methods  of 
enquiry  which  are  found  available  in  dealing  with 
natural  truth. 

Some  years  ago,  when  the  spirits  were  particularly 
active  in  this  country,  Faraday  was  invited,  or  rather 
entreated,  by  one  of  his  friends  to  meet  and  question 
them.  He  had,  however,  already  made  their  acquaint- 
ance, and  did  not  wish  to  renew  it.  I  had  not  been 
so  privileged,  and  he  therefore  kindly  arranged  a  trans- 
fer of  the  invitation  to  me.  The  spirits  themselves 
named  the  time  of  meeting,  and  I  was  conducted  to 
the  place  at  the  day  and  hour  appointed. 

Absolute  unbelief  in  the  facts  was  by  no  means  my 
condition  of  mind.  On  the  contrary,  I  thought  it  prob- 
able that  some  physical  principle,  not  evident  to  the 
spiritualists  themselves,  might  underlie  their  manifes- 
tations. Extraordinary  effects  are  produced  by  the 
accumulation  of  small  impulses.  Galileo  set  a  heavy 
pendulum  in  motion  by  the  well-timed  puffs  of  his 
breath.  Ellicot  set  one  clock  going  by  the  ticks  of 
another,  even  when  the  two  clocks  were  separated  by  a 
wall.  Preconceived  notions  can,  moreover,  vitiate,  to 
444 


SCIENCE    AND   THE    'SPIRITS.'  445 

an  extraordinary  degree,  the  testimony  of  even  vera- 
cious persons.  Hence  my  desire  to  witness  those  ex- 
traordinary phenomena,  the  existence  of  which  seemed 
placed  beyond  a  doubt  by  the  known  veracity  of  those 
who  had  witnessed  and  described  them.  The  meeting 
took  place  at  a  private  residence  in  the  neighbourhood 
of  London.  My  host,  his  intelligent  wife,  and  a  gentle- 
man who  may  be  called  X.,  were  in  the  house  when  I 
arrived.  I  was  informed  that  the  '  medium '  had  not 
yet  made  her  appearance;  that  she  was  sensitive,  and 
might  resent  suspicion.  It  was  therefore  requested 
that  the  tables  and  chairs  should  be  examined  before 
her  arrival,  in  order  to  be  assured  that  there  was  no 
trickery  in  the  furniture.  This  was  done;  and  I  then 
first  learned  that  my  hospitable  host  had  arranged  that 
the  seance  should  be  a  dinner-party.  This  was  to  me 
an  unusual  form  of  investigation;  but  I  accepted  it, 
as  one  of  the  accidents  of  the  occasion. 

The  ( medium '  arrived — a  delicate-looking  young 
lady,  who  appeared  to  have  suffered  much  from  ill- 
health.  I  took  her  to  dinner  and  sat  close  beside  her. 
Facts  were  absent  for  a  considerable  time,  a  series  of 
very  wonderful  narratives  supplying  their  place.  The 
duty  of  belief  on  the  testimony  of  witnesses  was  fre- 
quently insisted  on.  X.  appeared  to  be  a  chosen  spir- 
itual agent,  and  told  us  many  surprising  things.  He 
affirmed  that,  when  he  took  a  pen  in  his  hand,  an  influ- 
ence ran  from  his  shoulder  downwards,  and  impelled 
him  to  write  oracular  sentences.  I  listened  for  a  time, 
offering  no  observation.  *  And  now/  continued  X., 
'  this  power  has  so  risen  as  to  reveal  to  me  the  thoughts 
of  others.  Only  this  morning  I  told  a  friend  what  he 
was  thinking  of,  and  what  he  intended  to  do  during 
the  day/  Here,  I  thought,  is  something  that  can  be 
at  once  tested.  I  said  immediately  to  X.:  *  If  you  wish 


446  FEAGMENTS    OF    SCIENCE. 

to  win  to  your  cause  an  apostle,  who  will  proclaim  your 
principles  to  the  world  from  the  housetop,  tell  me  what 
I  am  now  thinking  of.'  X.  reddened,  and  did  not  tell 
me  my  thought. 

Some  time  previously  I  had  visited  Baron  Eeichen- 
bach,  in  Vienna,  and  I  now  asked  the  young  lady  who 
sat  beside  me,  whether  she  could  see  any  of  the  curious 
things  which  he  describes — the  light  emitted  by  crys- 
tals, for  example?  Here  is  the  conversation  which 
followed,  as  extracted  from  my  notes,  written  on  the 
day  following  the  seance. 

Medium. — '  Oh,  yes;  but  I  see  light  around  all 
bodies.' 

J. — '  Even  in  perfect  darkness? ' 

Medium. — '  Yes;  I  see  luminous  atmospheres  round 
all  people.  The  atmosphere  which  surrounds  Mr.  E.  C. 
would  fill  this  room  with  light.' 

I. — '  You  are  aware  of  the  effects  ascribed  by  Baron 
Eeichenbach  to  magnets? ' 

Medium. — '  Yes;  but  a  magnet  makes  me  terribly 
ill.' 

I. — '  Am  I  to  understand  that,  if  this  room  were 
perfectly  dark,  you  could  tell  whether  it  contained  a 
magnet,  without  being  informed  of  the  fact?' 

Medium. — '  I  should  know  of  its  presence  on  en- 
tering the  room.' 

I.— '  How? ' 

Medium. — <  I  should  be  rendered  instantly  ill.' 

/.— '  How  do  you  feel  to-day? ' 

Medium. — '  Particularly  well;  I  have  not  been  so 
well  for  months.' 

I. — '  Then,  may  I  ask  you  whether  there  is,  at  the 
present  moment,  a  magnet  in  my  possession? ' 

The  young  lady  looked  at  me,  blushed,  and  stam- 
mered, 


SCIENCE    AND    THE    'SPIRITS.'  447 

'  No;  I  am  not  en  rapport  with  you/ 

I  sat  at  her  right  hand,  and  a  left-hand  pocket, 
within  six  inches  of  her  person,  contained  a  magnet. 

Our  host  here  deprecated  discussion,  as  it '  exhaust- 
ed the  medium.'  The  wonderful  narratives  were  re- 
sumed; but  I  had  narratives  of  my  own  quite  as  won- 
derful. These  spirits,  indeed,  seemed  clumsy  creations, 
compared  with  those  with  which  my  own  work  had 
made  me  familiar.  - 1  therefore  began  to  match  the 
wonders  related  to  me  by  other  wonders.  A  lady  pres- 
ent discoursed  on  spiritual  atmospheres,  which  she 
could  see  as  beautiful  colours  when  she  closed  her  eyes. 
I  professed  myself  able  to  see  similar  colours,  and,  more 
than  that,  to  be  able  to  see  the  interior  of  my  own 
eyes.  The  medium  affirmed  that  she  could  see  actual 
waves  of  light  coming  from  the  sun.  I  retorted  that 
men  of  science  could  tell  the  exact  number  of  waves 
emitted  in  a  second,  and  also  their  exact  length.  The 
medium  spoke  of  the  performances  of  the  spirits  on 
musical  instruments.  I  said  that  such  performance 
was  gross,  in  comparison  with  a  kind  of  music  which 
had  been  discovered  some  time  previously  by  a  scien- 
tific man.  Standing  at  a  distance  of  twenty  feet  from 
a  jet  of  gas,  he  could  command  the  flame  to  emit  a 
melodious  note;  it  would  obey,  and  continue  its  song 
for  hours.  So  loud  was  the  music  emitted  by  the  gas- 
flame,  that  it  might  be  heard  by  an  assembly  of  a 
thousand  people.  These  were  acknowledged  to  be  as 
great  marvels  as  any  of  those  of  spiritdom.  The  spirits 
were  then  consulted,  and  I  was  pronounced  to  be  a 
first-class  medium. 

During  this  conversation  a  low  knocking  was  heard 
from  time  to  time  under  the  table.  These,  I  was  told, 
were  the  spirits'  knocks.  I  was  informed  that  one 
knock,  in  answer  to  a  question,  meant  '  No; '  that  two 


448  FRAGMENTS    OF    SCIENCE. 

knocks  meant  '  Not  yet,'  and  that  three  knocks  meant 
*  Yes.'  In  answer  to  a  question  whether  I  was  a  medi- 
um, the  response  was  three  brisk  and  vigorous  knocks. 
I  noticed  that  the  knocks  issued  from  a  particular 
locality,  and  therefore  requested  the  spirits  to  be  good 
enough  to  answer  from  another  corner  of  the  table. 
They  did  not  comply;  but  I  was  assured  that  they 
would  do  it,  and  much  more,  by-and-by.  The  knocks 
continuing,  I  turned  a  wine-glass  upside  down,  and 
placed  my  ear  upon  it,  as  upon  a  stethoscope.  The 
spirits  seemed  disconcerted  by  the  act;  they  lost  their 
playfulness,  and  did  not  recover  it  for  a  considerable 
time. 

Somewhat  weary  of  the  proceedings,  I  once  threw 
myself  back  against  my  chair  and  gazed  listlessly  out 
of  the  window.  While  thus  engaged,  the  table  was 
rudely  pushed.  Attention  was  drawn  to  the  wine,  still 
oscillating  in  the  glasses,  and  I  was  asked  whether  that 
was  not  convincing.  I  readily  granted  the  fact  of 
motion,  and  began  to  feel  the  delicacy  of  my  position. 
There  were  several  pairs  of  arms  upon  the  table,  and 
several  pairs  of  legs  under  it;  but  how  was  I,  without 
offence,  to  express  the  conviction  which  I  really  enter- 
tained? To  ward  off  the  difficulty,  I  again  turned  a 
wine-glass  upside  down  and  rested  my  ear  upon  it. 
The  rim  of  the  glass  was  not  level,  and  my  hair,  on 
touching  it,  caused  it  to  vibrate,  and  produce  a  pecul- 
iar buzzing  sound.  A  perfectly  candid  and  warm- 
hearted old  gentleman  at  the  opposite  side  of  the  table, 
whom  I  may  call  A.,  drew  attention  to  the  sound,  and 
expressed  his  entire  belief  that  it  was  spiritual.  I, 
however,  informed  him  that  it  was  the  moving  hair 
acting  on  the  glass.  The  explanation  was  not  well 
received;  and  X.,  in  a  tone  of  severe  pleasantry,  de- 
manded whether  it  was  the  hair  that  had  moved  the 


SCIENCE   AND   THE    'SPIRITS.'  449 

table.  The  promptness  of  my  negative  probably  satis- 
fied him  that  my  notion  was  a  very  different  one. 

The  superhuman  power  of  the  spirits  was  next 
dwelt  upon.  The  strength  of  man,  it  was  stated,  was 
unavailing  in  opposition  to  theirs.  No  human  power 
could  prevent  the  table  from  moving  when  they  pulled 
it.  During  the  evening  this  pulling  of  the  table  oc- 
curred, or  rather  was  attempted,  three  times.  Twice 
the  table  moved  when  my  attention  was  withdrawn 
from  it;  on  a  third  occasion,  I  tried  whether  the  act 
could  be  prcfvoked  by  an  assumed  air  of  inattention. 
Grasping  the  table  firmly  between  my  knees,  I  threw 
myself  back  in  the  chair,  and  waited,  with  eyes  fixed 
on  vacancy,  for  the  pull.  It  came.  For  some  seconds 
it  was  pull  spirit,  hold  muscle;  the  muscle,  however, 
prevailed,  and  the  table  remained  at  rest.  Up  to  the 
present  moment,  this  interesting  fact  is  known  only  to 
the  particular  spirit  in  question  and  myself. 

A  species  of  mental  scene-painting,  with  which  my 
own  pursuits  had  long  rendered  me  familiar,  was  em- 
ployed to  figure  the  changes  and  distribution  of  spirit- 
ual power.  The  spirits,  it  was  alleged,  were  provided 
with  atmospheres,  which  combined  with  and  interpene- 
trated each  other,  and  considerable  ingenuity  was 
shown  in  demonstrating  the  necessity  of  time  in  effect- 
ing the  adjustment  of  the  atmospheres.  A  re-arrange- 
ment of  our  positions  was  proposed  and  carried  out; 
and  soon  afterwards  my  attention  was  drawn  to  a 
scarcely  sensible  vibration  on  the  part  of  the  table. 
Several  persons  were  leaning  on  the  table  at  the  time, 
and  I  asked  permission  to  touch  the  medium's  hand. 
'  Oh!  I  know  I  tremble,'  was  her  reply.  Throwing  one 
leg  across  the  other,  I  accidentally  nipped  a  muscle, 
and  produced  thereby  an  involuntary  vibration  of  the 
free  leg.  This  vibration,  I  knew,  must  be  communi- 


450  FRAGMENTS    OF    SCIENCE. 

cated  to  the  floor,  and  thence  to  the  chairs  of  all  pres- 
ent. I  therefore  intentionally  promoted  it.  My  atten- 
tion was  promptly  drawn  to  the  motion;  and  a  gen- 
tleman beside  me,  whose  value  as  a  witness  I  was 
particularly  desirous  to  test,  expressed  his  belief  that 
it  was  out  of  the  compass  of  human  power  to  produce 
so  strange  a  tremor.  *  I  believe/  he  added,  earnestly, 
*  that  it  is  entirely  the  spirits'  work.'  '  So  do  I/  added, 
with  heat,  the  candid  and  warm-hearted  old  gentleman 
A.  '  Why,  sir/  he  continued,  '  I  feel  them  at  this 
moment  shaking  my  chair.'  I  stopped  the  motion  of 
the  leg.  'Now,  sir/  A.  exclaimed,  'they  are  gone.' 
I  began  again,  and  A.  once  more  affirmed  their  pres- 
ence. I  could,  however,  notice  that  there  were  doubt- 
ers present,  who  did  not  quite  know  what  to  think  of 
the  manifestations.  I  saw  their  perplexity;  and,  as 
there  was  sufficient  reason  to  believe  that  the  disclosure 
of  the  secret  would  simply  provoke  anger,  I  kept  it  to 
myself. 

Again  a  period  of  conversation  intervened,  during 
which  the  spirits  became  animated.  The  evening  was 
confessedly  a  dull  one,  but  matters  appeared  to  brighten 
towards  its  close.  The  spirits  were  requested  to  spell 
the  name  by  which  I  was  known  in  the  heavenly  world. 
Our  host  commenced  repeating  the  alphabet,  and  when 
he  reached  the  letter  '  P '  a  knock  was  heard.  He 
began  again,  and  the  spirits  knocked  at  the  letter  '  0.' 
I  was  puzzled,  but  waited  for  the  end.  The  next  letter 
knocked  down  was  '  E/  I  laughed,  and  remarked  that 
the  spirits  were  going  to  make  a  poet  of  me.  Ad- 
monished for  my  levity,  I  was  informed  that  the  frame 
of  mind  proper  for  the  occasion  ought  to  have  been 
superinduced  by  a  perusal  of  the  Bible  immediately 
before  the  seance.  The  spelling,  however,  went  on, 
and  sure  enough  I  came  out  a  poet.  But  matters  did 


SCIENCE    AND    THE    'SPIRITS.'  451 

not  end  here.  Our  host  continued  his  repetition  of 
the  .alphabet,  and  the  next  letter  of  the  name  proved 
to  be  '  0.'  Here  was  manifestly  an  unfinished  word; 
and  the  spirits  were  apparently  in  their  most  communi- 
cative mood.  The  knocks  came  from  under  the  table, 
but  no  person  present  evinced  the  slightest  desire  to 
look  under  it.  I  asked  whether  I  might  go  under- 
neath; the  permission  was  granted;  so  I  crept  under 
the  table.  Some  tittered;  but  the  candid  old  A.  ex- 
claimed, '  He  has  a  right  to  look  into  the  very  dregs  of 
it,  to  convince  himself/  Having  pretty  well  assured 
myself  that  no  sound  could  be  produced  under  the 
table  without  its  origin  being  revealed,  I  requested  our 
host  to  continue  his  questions.  He  did  so,  but  in  vain. 
He  adopted  a  tone  of  tender  entreaty;  but  the  '  dear 
spirits '  had  become  dumb  dogs,  and  refused  to  be 
entreated.  I  continued  under  that  table  for  at  least  a 
quarter  of  an  hour,  after  which,  with  a  feeling  of 
despair  as  regards  the  prospects  of  humanity  never  be- 
fore experienced,  I  regained  my  chair.  Once  there, 
the  spirits  resumed  their  loquacity,  and  dubbed  me 
'  Poet  of  Science/ 

This,  then,  is  the  result  of  an  attempt  made  by  a 
scientific  man  to  look  into  these  spiritual  phenomena. 
It  is  not  encouraging;  and  for  this  reason.  The  pres- 
ent promoters  of  spiritual  phenomena  divide  them- 
selves into  two  classes,  one  of  which  needs  no  demon- 
stration, while  the  other  is  beyond  the  reach  of  proof. 
The  victims  like  to  believe,  and  they  do  not  like  to  be 
undeceived.  Science  is  perfectly  powerless  in  the  pres- 
ence of  this  frame  of  mind.  It  is,  moreover,  a  state 
perfectly  compatible  with  extreme  intellectual  subtlety 
and  a  capacity  for  devising  hypotheses  which  only 
require  the  hardihood  engendered  by  strong  conviction, 
or  by  callous  mendacity,  to  render  them  impregnable. 


452  FRAGMENTS    OF    SCIENCE. 

The  logical  feebleness  of  science  is  not  sufficiently 
borne  in  mind.  It  keeps  down  the  weed  of  supersti- 
tion, not  by  logic  but  by  slowly  rendering  the  mental 
soil  unfit  for  its  cultivation.  When  science  appeals  to 
uniform  experience,  the  spiritualist  will  retort,  '  How 
do  you  know  that  a  uniform  experience  will  continue 
uniform?  You  tell  me  that  the  sun  has  risen  for  six 
thousand  years:  that  is  no  proof  that  it  will  rise  to- 
morrow; within  the  next  twelve  hours  it  may  be  puffed 
out  by  the  Almighty.'  Taking  this  ground,  a  man 
may  maintain  the  story  of  '  Jack  and  the  Beanstalk '  in 
the  face  of  all  the  science  in  the  world.  You  urge,  in 
vain,  that  science  has  given  us  all  the  knowledge  of 
the  universe  which  we  now  possess,  while  spiritualism 
has  added  nothing  to  that  knowledge.  The  drugged 
soul  is  beyond  the  reach  of  reason.  It  is  in  vain  that 
impostors  are  exposed,  and  the  special  demon  cast  out. 
He  has  but  slightly  to  change  his  shape,  return  to  his 
house,  and  find  it '  empty,  swept,  and  garnished.' 


Since  the  time  when  the  foregoing  remarks  were 
written  I  have  been  more  than  once  among  the  spirits, 
at  their  own  invitation.  They  do  not  improve  on  ac- 
quaintance. Surely  no  baser  delusion  ever  obtained 
dominance  over  the  weak  mind  of  man. 


END   OF   THE   FIRST   VOLUME. 


A     000  024  989     6 


